compositor-drm: Don't test render-only atomic configuration

In the RENDERER_ONLY state proposal mode, we don't actually have a
viable configuration to test, because we won't get a renderer buffer
until after assign_planes - where we're called from - has completed.

This can result in us trying to test a configuration with the CRTC and
connectors active, but no planes active, which the kernel can
legitimately fail.

If we're working in renderer-only mode, just return the state we have
without trying to test it first, and let the kernel fill it in later.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derek.foreman.samsung@gmail.com>

libweston/compositor-drm.c

@@ -3488,6 +3488,11 @@ drm_output_propose_state(struct weston_output *output_base,
 	pixman_region32_fini(&renderer_region);
 	pixman_region32_fini(&occluded_region);
 
+	/* In renderer-only mode, we can't test the state as we don't have a
+	 * renderer buffer yet. */
+	if (mode == DRM_OUTPUT_PROPOSE_STATE_RENDERER_ONLY)
+		return state;
+
 	/* Check to see if this state will actually work. */
 	ret = drm_pending_state_test(state->pending_state);
 	if (ret != 0)