drm/xen: fix passing zero to 'PTR_ERR' warning

Fix a static code checker warning:
    drivers/gpu/drm/xen/xen_drm_front.c:404 xen_drm_drv_dumb_create()
    warn: passing zero to 'PTR_ERR'

Signed-off-by: Ding Xiang <dingxiang@cmss.chinamobile.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1585562347-30214-1-git-send-email-dingxiang@cmss.chinamobile.com