When this flag is set, we program the hardware to execute the flip
during horizontal blank (i.e. for the next scanline) instead of during
vertical blank (i.e. for the next frame).

Ported from radeon commit:
drm/radeon: Support DRM_MODE_PAGE_FLIP_ASYNC

v2: drop DAL change for upstream
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>