drm/i915: Remove intel_finish_page_flip_plane.

This function is duplicated with intel_finish_page_flip,
and is only ever used from planes that could use the
other function anyway.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463490484-19540-4-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>