drm/i915: Remove commit_plane function pointer.

With sprites, cursors and primary planes taking the atomic state
this is now unused. It's removed in a separate commit to allow
a revert.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452164052-21752-8-git-send-email-maarten.lankhorst@linux.intel.com


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>