Mostly the same as the existing page_flip hook, but takes an additional
parameter specifying the target vertical blank period when the flip
should take effect.

v2:
* Add curly braces around else statement corresponding to an if block
  with curly braces (Alex Deucher)
* Call drm_crtc_vblank_put in the error case (Daniel Vetter)
* Clarify entry point documentation comment (Daniel Vetter)
Acked-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NMichel Dänzer <michel.daenzer@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>