compositor-drm: Only request vblank events if needed

If the sprite is disabled and we're not enabling it on the next frame,
nothing is done in the vblank handler, so there's no need to ask for a
vblank event.

src/compositor-drm.c

@@ -391,7 +391,8 @@ drm_output_repaint(struct weston_output *output_base,
 			.request.sequence = 1,
 		};
 
-		if (!drm_sprite_crtc_supported(output_base, s->possible_crtcs))
+		if ((!s->fb_id && !s->pending_fb_id) ||
+		    !drm_sprite_crtc_supported(output_base, s->possible_crtcs))
 			continue;
 
 		ret = drmModeSetPlane(compositor->drm.fd, s->plane_id,