compositor-drm: Make drm_output_check_scanout_format() work for all formats

This used to work only for ARGB8888 and XRGB8888 buffers, but this lets
us support pageflipping to any client buffer as long as it matches the
framebuffer format.

src/compositor-drm.c

@@ -413,32 +413,31 @@ static uint32_t
 drm_output_check_scanout_format(struct drm_output *output,
 				struct weston_surface *es, struct gbm_bo *bo)
 {
+	struct drm_compositor *c =
+		(struct drm_compositor *) output->base.compositor;
 	uint32_t format;
 	pixman_region32_t r;
 
 	format = gbm_bo_get_format(bo);
 
-	switch (format) {
-	case GBM_FORMAT_XRGB8888:
-		return format;
-	case GBM_FORMAT_ARGB8888:
-		/* We can only scanout an ARGB buffer if the surface's
-		 * opaque region covers the whole output */
+	if (format == GBM_FORMAT_ARGB8888) {
+		/* We can scanout an ARGB buffer if the surface's
+		 * opaque region covers the whole output, but we have
+		 * to use XRGB as the KMS format code. */
 		pixman_region32_init(&r);
 		pixman_region32_subtract(&r, &output->base.region,
 					 &es->opaque);
 
 		if (!pixman_region32_not_empty(&r))
 			format = GBM_FORMAT_XRGB8888;
-		else
-			format = 0;
 
 		pixman_region32_fini(&r);
+	}
 
+	if (c->format == format)
 		return format;
-	default:
-		return 0;
-	}
+
+	return 0;
 }
 
 static struct weston_plane *