drm/radeon: Support DRM_MODE_PAGE_FLIP_ASYNC

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).

Currently this is only supported on ASICs which have a page flip
completion interrupt (>= R600), and only if the use_pflipirq parameter
has value 2 (the default).

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>