backend/drm: make commits without a buffer blocking

The wlr_output API requires compositors to wait for wlr_output.frame
before submitting a new buffer. However, compositors can perform a
commit which doesn't involve a buffer anytime.

If the commit is a modeset, we set DRM_MODE_ATOMIC_ALLOW_MODESET and
block until the commit is done. If it isn't, we currently always
perform a non-blocking commit. This is an issue because a previous
page-flip might still be in flight kernel-side, returning EBUSY.

Fix this by using blocking commits when a buffer isn't submitted by
the compositor.

Closes: https://github.com/swaywm/sway/issues/6962
References: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/2239

backend/drm/atomic.c

@@ -264,7 +264,12 @@ static bool atomic_crtc_commit(struct wlr_drm_connector *conn,
 	}
 	if (modeset) {
 		flags |= DRM_MODE_ATOMIC_ALLOW_MODESET;
-	} else if (!test_only) {
+	} else if (!test_only && (state->base->committed & WLR_OUTPUT_STATE_BUFFER)) {
+		// The wlr_output API requires non-modeset commits with a new buffer to
+		// wait for the frame event. However compositors often perform
+		// non-modesets commits without a new buffer without waiting for the
+		// frame event. In that case we need to make the KMS commit blocking,
+		// otherwise the kernel will error out with EBUSY.
 		flags |= DRM_MODE_ATOMIC_NONBLOCK;
 	}