modesetting: Consider RandR primary output for selectioh of sync crtc.
The "sync crtc" is the crtc used to drive the display timing of a drawable under DRI2 and DRI3/Present. If a drawable intersects multiple video outputs, then normally the crtc is chosen which has the largest intersection area with the drawable. If multiple outputs / crtc's have exacty the same intersection area then the crtc chosen was simply the first one with maximum intersection. Iow. the choice was random, depending on plugging order of displays. This adds the ability to choose a preferred output in such a tie situation. The RandR output marked as "primary output" is chosen on such a tie. This new behaviour and its implementation is consistent with other video ddx drivers. See amdgpu-ddx, ati-ddx and nouveau-ddx for reference. This commit is a straightforward port from amdgpu-ddx. Signed-off-by: Mario Kleiner <email@example.com>