drm: Add drm_crtc_vblank_waitqueue()

Add a small static inline helper to grab the vblank wait queue based on
the drm_crtc.

This is useful for drivers to do internal vblank waits using
wait_event() & co.

v2: Pimp commit message (Daniel)
    Add kernel doc (Daniel)

Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>