This is a straightforward port of a patch with the same name
"modesetting: Add option for non-vsynced flips for "secondary"
outputs." from X-Server master / X-Server 21.1. See server MR 742.
The description below is therefore identical to that X-Server commit:

Whenever an unredirected fullscreen window uses pageflipping for a
DRI3/Present PresentPixmap() operation and the X-Screen has more than
one active output, multiple crtc's need to execute pageflips. Only
after the last flip has completed can the PresentPixmap operation
as a whole complete.

If a sync_flip is requested for the present, then the current
implementation will synchronize each pageflip to the vblank of
its associated crtc. This provides tear-free image presentation
across all outputs, but introduces a different artifact, if not
all outputs run at the same refresh rate with perfect synchrony:
The slowest output throttles the presentation rate, and present
completion is delayed to flip completion of the "latest" output
to complete. This means degraded performance, e.g., a dual-display
setup with a 144 Hz monitor and a 60 Hz monitor will always be
throttled to at most 60 fps. It also means non-constant present
rate if refresh cycles drift against each other, creating complex
"beat patterns", tremors, stutters and periodic slowdowns - quite
irritating!

Such a scenario will be especially annoying if one uses multiple
outputs in "mirror mode" aka "clone mode". One output will usually
be the "production output" with the highest quality and fastest
display attached, whereas a secondary mirror output just has a
cheaper display for monitoring attached. Users care about perfect
and perfectly timed tear-free presentation on the "production output",
but cares less about quality on the secondary "mirror output". They
are willing to trade quality on secondary outputs away in exchange
for better presentation timing on the "production output".

One example use case for such production + monitoring displays are
neuroscience / medical science applications where one high quality
display device is used to present visual animations to test subjects
or patients in a fMRI scanner room (production display), whereas
an operator monitors the same visual animations from a control room
on a lower quality display. Presentation timing needs to be perfect,
and animations high-speed and tear-free for the production display,
whereas quality and timing don't matter for the monitoring display.

This commit gives users the option to choose such a trade-off as
opt-in:

It adds a new boolean option "AsyncFlipSecondaries" to the device section
of xorg.conf. If this option is specified as true, then DRI3 pageflip
behaviour changes as follows:

1. The "reference crtc" for a windows PresentPixmap operation does a
   vblank synced flip, or a DRM_MODE_PAGE_FLIP_ASYNC non-synchronized
   flip, as requested by the caller, just as in the past. Typically
   flips will be requested to be vblank synchronized for tear-free
   presentation. The "reference crtc" is the one chosen by the caller
   to drive presentation timing (as specified by PresentPixmap()'s
   "target_msc", "divisor", "remainder" parameters and implemented by
   vblank events) and to deliver Present completion timestamps (msc
   and ust) extracted from its pageflip completion event.

2. All other crtc's, which also page-flip in a multi-display configuration,
   will try to flip with DRM_MODE_PAGE_FLIP_ASYNC, ie. immediately and
   not synchronized to vblank. This allows the PresentPixmap operation
   to complete with little delay compared to a single-display present,
   especially if the different crtc's run at different video refresh
   rates or their refresh cycles are not perfectly synchronized, but
   drift against each other. The downside is potential tearing artifacts
   on all outputs apart from the one of the "reference crtc".

Successfully tested on a AMD gpu with single-display and dual-display
setups, and with single-X-Screen as well as dual-X-Screen "ZaphodHeads"
configurations.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>