randr: Add ability to turn PRIME sync off

Adds an output parameter to disable PRIME synchronization.

Output parameter is created when the user calls 'xrandr
--setprovideroutputsource <sink> <source>' to prevent polluting output
parameters of non-PRIME configurations.

Defaults to on, so if the user wants PRIME synchronization they don't need
to do anything.

If the user wishes to disable PRIME synchronization when they first set up
PRIME, they can run 'xrandr --output <output> --set "PRIME Synchronization"
0' after running 'xrandr --setprovideroutputsource <sink> <source>', but
before 'xrandr --auto'.

If the user wishes to enable or disable PRIME synchronization after PRIME has
already been set up, they can run 'xrandr --output <output> --set "PRIME
Synchronization" <0 or 1>' at any time, xrandr will trigger a modeset, which
will tear down and setup PRIME in the configuration they requested on CRTCs
associated with that output.

randrstr.h:
    Add central definition of the output property name.

rrcrtc.c:
    Add function rrGetPixmapSharingSyncProp() to query the status of the
    output property.

    Add function rrSetPixmapSharingSyncProp() to set the output property.

    Add 'sync' parameter to rrSetupPixmapSharing(), which when false will
    use single buffering even if the required ABI functions are supported.
    Changes rrSetupPixmapSharing() to only report an error if falling back
    to single buffering when the user requested synchronization.

    Change RRCrtcSet() to use rrPixmapSharingSyncProp() to query the status
    of the output property and feed it into rrSetupPixmapSharing() using
    the 'sync' parameter.

rrprovider.c:
    Add RR(Init/Fini)PrimeSyncProps(), functions to create and destroy the
    PRIME synchronization output property.

    Add a call to RRInitPrimeSyncProps() in
    ProcRRSetProviderOutputSource(), such that the output property is
    created when the user requests PRIME.

    Add a call to RRFiniPrimeSyncProps() in RRProviderDestroy().

v1: Initial commit
v2: Unchanged
v3: Add /* TODO */ for handling different sources with different outputs
    Make rrSetupPixmapSharing() set the output property to 0 if it has to fall
    back, to avoid user confusion.
    Make rr(Get)PixmapSharingSyncProp() check the current value if there isn't a
    pending value
v4: Unchanged
v5: Unchanged
v6: Rebase onto ToT
v7: Unchanged

Signed-off-by: Alex Goins <agoins@nvidia.com>