drm: Use PTR_ERR_OR_ZERO in drm_fb_cma_fbdev_init()

Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1539176563-144779-1-git-send-email-yuehaibing@huawei.com