modesetting: Implement ms_covering_randr_crtc() for ms_present_get_crtc()

ms_present_get_crtc() returns an RRCrtcPtr, but derives it from a xf86CrtcPtr
found via ms_dri2_crtc_covering_drawable()=>ms_covering_crtc(). As a result, it
depends on all associated DIX ScreenRecs having an xf86CrtcConfigPtr DDX
private.

Some DIX ScreenRecs don't have an xf86CrtcConfigPtr DDX private, but do have an
rrScrPrivPtr DDX private. Given that we can derive all of the information we
need from RandR, we can support these screens by avoiding the use of xf86Crtc.
This change implements an RandR-based path for ms_present_get_crtc(), allowing
drawables to successfully fall back to syncing to the primary output, even if
the slave doesn't have an xf86CrtcConfigPtr DDX private.

Without this change, if a slave doesn't have an xf86CrtcConfigPtr DDX private,
drawables will fall back to 1 FPS if they overlap an output on that slave.

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