backend/drm: fix EINVAL atomic commits after VT switch

The drm_connector_commit_state() call in handle_session_active()
was not resulting in any atomic commit, because it didn't match any
of the if branches: active = true, no new buffer was committed,
and adaptive sync/gamma LUT were unchanged. Thus the commit was a
no-op.

Later on, when the compositor performs regular page-flips, the
kernel would return EINVAL indicating that a modeset was needed.

Rework the logic to use a non-blocking page-flip commit if a buffer
was committed, and use a blocking commit if the connector is on or
is being disabled. The only case where we should skip the atomic
commit is when disabling (active = false) an already-disabled
connector (conn->crtc == NULL).

Note, 6936e163b5 ("backend/drm: short-circuit no-op commits")
has introduced early returns for other situations where we don't
need to perform an atomic commit (e.g. updating scale or transform
of an output).

Fixes: f216e97983 ("backend/drm: drop drm_connector_set_mode()")
Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3432

backend/drm/drm.c

@@ -592,18 +592,11 @@ bool drm_connector_commit_state(struct wlr_drm_connector *conn,
 		}
 	}
 
-	if (!pending.active) {
+	if (pending.base->committed & WLR_OUTPUT_STATE_BUFFER) {
-		if (conn->crtc != NULL && !drm_crtc_commit(conn, &pending, 0, false)) {
-			return false;
-		}
-	} else if (pending.base->committed & WLR_OUTPUT_STATE_BUFFER) {
 		if (!drm_crtc_page_flip(conn, &pending)) {
 			return false;
 		}
-	} else if (pending.base->committed & (WLR_OUTPUT_STATE_ADAPTIVE_SYNC_ENABLED |
+	} else if (pending.active || conn->crtc != NULL) {
-			WLR_OUTPUT_STATE_GAMMA_LUT)) {
-		assert(conn->crtc != NULL);
-		// TODO: maybe request a page-flip event here?
 		if (!drm_crtc_commit(conn, &pending, 0, false)) {
 			return false;
 		}