With the previous change, it's safe to let page flips take effect
anytime during a vertical blank period.

This can avoid delaying a flip by a frame in some cases where we get to
amdgpu_flip_work_func -> adev->mode_info.funcs->page_flip during a
vertical blank period.
Acked-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NMichel Dänzer <michel.daenzer@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>