mipi_dsi_device_register_full() never returns NULL pointer, it
will return ERR_PTR() when it fails, so replace the check with
IS_ERR().

Fixes: 0993234a ("drm/panel: Add driver for Novatek NT36523")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20231129090715.856263-1-yangyingliang@huaweicloud.comSigned-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231129090715.856263-1-yangyingliang@huaweicloud.com