drm/atomic: Add target_vblank support in atomic helpers (v2)

Allows usage of the new page_flip_target hook for drivers implementing
the atomic path.
Provides default atomic helper for the new hook.

v2:
Update code sharing logic between exsiting and the new flip hooks.
Improve kerneldoc.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1483735180-4173-1-git-send-email-Andrey.Grodzovsky@amd.com