drm: Fix kernel doc for DRM_MODE_PROP_IMMUTABLE

This patch explains the DRM_MODE_PROP_IMMUTABLE flag a bit better
by telling which function to call if kernel wants to update
drm object's immutable properties.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181002215055.8652-1-manasi.d.navare@intel.com

include/drm/drm_property.h

@@ -153,7 +153,8 @@ struct drm_property {
 	 *     userspace. The kernel is allowed to update the value of these
 	 *     properties. This is generally used to expose probe state to
 	 *     userspace, e.g. the EDID, or the connector path property on DP
-	 *     MST sinks.
+	 *     MST sinks. Kernel can update the value of an immutable property
+	 *     by calling drm_object_property_set_value().
 	 */
 	uint32_t flags;