drm/qxl: Add check for drm_cvt_mode

Add check for the return value of drm_cvt_mode() and return the error if
it fails in order to avoid NULL pointer dereference.

Fixes: 1b043677 ("drm/qxl: add qxl_add_mode helper function")
Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Reviewed-by: Heng Qi <hengqi@linux.alibaba.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240621071031.1987974-1-nichen@iscas.ac.cn

drivers/gpu/drm/qxl/qxl_display.c

@@ -236,6 +236,9 @@ static int qxl_add_mode(struct drm_connector *connector,
 		return 0;
 
 	mode = drm_cvt_mode(dev, width, height, 60, false, false, false);
+	if (!mode)
+		return 0;
+
 	if (preferred)
 		mode->type |= DRM_MODE_TYPE_PREFERRED;
 	mode->hdisplay = width;