drm/radeon: Provide page_flip_target hook

Now we can program a flip during a vertical blank period, if it's the
one targeted by the flip (or a later one). This allows simplifying
radeon_flip_work_func considerably.

Acked-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>