drm: Reject replacing property enum values

If the property already has the enum value WARN and bail.
Replacing enum values doesn't make sense to me.

Throw out the pointless list_empty() while at it.

Cc: Daniel Vetter <daniel@ffwll.ch>
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180306164849.2862-1-ville.syrjala@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_property.c

@@ -390,14 +390,9 @@ int drm_property_add_enum(struct drm_property *property, int index,
 			(value > 63))
 		return -EINVAL;
 
-	if (!list_empty(&property->enum_list)) {
 	list_for_each_entry(prop_enum, &property->enum_list, head) {
-			if (prop_enum->value == value) {
-				strncpy(prop_enum->name, name, DRM_PROP_NAME_LEN);
-				prop_enum->name[DRM_PROP_NAME_LEN-1] = '\0';
-				return 0;
-			}
-		}
+		if (WARN_ON(prop_enum->value == value))
+			return -EINVAL;
 	}
 
 	prop_enum = kzalloc(sizeof(struct drm_property_enum), GFP_KERNEL);