Mario Kleiner authored Oct 01, 2021 and Povilas Kanapickas committed Oct 08, 2021

In a setup with both VRR capable and non-VRR capable displays,
it was so far inconsistent if the driver would allow use of
VRR support or not, as "is_connector_vrr_capable" was set to
whatever the capabilities of the last added drm output were.
Iow. the plugging order of monitors determined the outcome.

Fix this: Now if at least one display is VRR capable, the driver
will treat an X-Screen as capable for VRR, plugging order no
longer matters.

Tested with a dual-display setup with one VRR monitor and one
non-VRR monitor. This is also beneficial with the new Option
"AsyncFlipSecondaries".

When we are at it, also add some so far missing description of
the "VariableRefresh" driver option, copied from amdgpu-ddx.

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