backend/session: fix KMS device filtering

As explained in [1], user-space should perform a drmModeGetResources
call to figure out whether a device supports KMS.

[1]: https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/127

backend/session/session.c

@@ -265,12 +265,20 @@ static struct wlr_device *open_if_kms(struct wlr_session *restrict session,
 		return NULL;
 	}
 
-	drmVersion *ver = drmGetVersion(dev->fd);
+	// The kernel errors out with EOPNOTSUPP if DRIVER_MODESET isn't set
-	if (!ver) {
+	drmModeRes *res = drmModeGetResources(dev->fd);
+	if (!res) {
+		if (errno != EOPNOTSUPP) {
+			wlr_log_errno(WLR_ERROR, "drmModeGetResources(%s) failed", path);
+		}
+		goto out_dev;
+	}
+	if (res->count_crtcs == 0) {
+		drmModeFreeResources(res);
 		goto out_dev;
 	}
+	drmModeFreeResources(res);
 
-	drmFreeVersion(ver);
 	return dev;
 
 out_dev: