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