drm: Make __drm_object_property_get_value() static

Make __drm_object_property_get_value() static to fix the following
sparse warning:
drm_mode_object.c:250:5: warning: symbol '__drm_object_property_get_value' was not declared. Should it be static?

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Fixes: 4a97a3da420b ("drm: Don't update property values for atomic drivers")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170901165328.24459-4-ville.syrjala@linux.intel.com
Reviewed-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/drm_mode_object.c

@@ -247,8 +247,9 @@ int drm_object_property_set_value(struct drm_mode_object *obj,
 }
 EXPORT_SYMBOL(drm_object_property_set_value);
 
-int __drm_object_property_get_value(struct drm_mode_object *obj,
-				  struct drm_property *property, uint64_t *val)
+static int __drm_object_property_get_value(struct drm_mode_object *obj,
+					   struct drm_property *property,
+					   uint64_t *val)
 {
 	int i;