compositor-drm: Forgot to use mode variable when using render only mode of composition

In patch 5d767416c1 we simplified a bit the way in which the
compositing mode was being printed with the purpose to improve
weston-debug.  It seems we forgot to use the mode when RENDER-only mode
is being used, so this patch fixes that.

Signed-off-by: Marius Vlad <marius.vlad0@gmail.com>

libweston/compositor-drm.c

@@ -3818,7 +3818,7 @@ drm_assign_planes(struct weston_output *output_base, void *repaint_data)
 	if (!state) {
 		mode = DRM_OUTPUT_PROPOSE_STATE_RENDERER_ONLY;
 		state = drm_output_propose_state(output_base, pending_state,
-						 DRM_OUTPUT_PROPOSE_STATE_RENDERER_ONLY);
+						 mode);
 	}
 
 	assert(state);