drm: Fix kerneldocs for drm_plane modifiers

Add the missing kerneldoc for modifiers and modifier_count.

Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114191021.15591-3-ville.syrjala@linux.intel.comReviewed-by: Alex Deucher <alexander.deucher@amd.com>

include/drm/drm_plane.h

@@ -474,6 +474,8 @@ enum drm_plane_type {
  * @format_types: array of formats supported by this plane
  * @format_count: number of formats supported
  * @format_default: driver hasn't supplied supported formats for the plane
+ * @modifiers: array of modifiers supported by this plane
+ * @modifier_count: number of modifiers supported
  * @old_fb: Temporary tracking of the old fb while a modeset is ongoing. Used by
  * 	drm_mode_set_config_internal() to implement correct refcounting.
  * @funcs: helper functions