drm: Remove unnecessary condition in drm_output_render reuse

This condition inside drm_output_render() checks if we can reuse the
existing renderer buffer for the primary plane; this occurs in
mixed-mode composition where a client buffer promoted to a plane has
changed, but the primary plane is unchanged.

We accomplish this by checking if there is no damage on the
primary/renderer plane, and then if there is already a renderer buffer
active on the primary plane: in that case, we can reuse the buffer we
already have.

There was a further condition checking if the width and height were
identical. This was designed to prevent against issues on mode changes.
However, runtime mode changes are already quite broken, and a mode
change will also cause damage on the full plane. We can simply remove
this condition.

Signed-off-by: Daniel Stone <daniels@collabora.com>

libweston/backend-drm/drm.c

@@ -364,14 +364,16 @@ drm_output_render(struct drm_output_state *state, pixman_region32_t *damage)
 	if (scanout_state->fb)
 		return;
 
+	/*
+	 * If we don't have any damage on the primary plane, and we already
+	 * have a renderer buffer active, we can reuse it; else we pass
+	 * the damaged region into the renderer to re-render the affected
+	 * area.
+	 */
 	if (!pixman_region32_not_empty(damage) &&
 	    scanout_plane->state_cur->fb &&
 	    (scanout_plane->state_cur->fb->type == BUFFER_GBM_SURFACE ||
-	     scanout_plane->state_cur->fb->type == BUFFER_PIXMAN_DUMB) &&
+	     scanout_plane->state_cur->fb->type == BUFFER_PIXMAN_DUMB)) {
-	    scanout_plane->state_cur->fb->width ==
-		output->base.current_mode->width &&
-	    scanout_plane->state_cur->fb->height ==
-		output->base.current_mode->height) {
 		fb = drm_fb_ref(scanout_plane->state_cur->fb);
 	} else if (b->use_pixman) {
 		fb = drm_output_render_pixman(state, damage);