drm/xen-front: checking for NULL instead of IS_ERR

drm_dev_alloc() returns error pointers, it never returns NULL.

Fixes: c575b7ee

 ("drm/xen-front: Add support for Xen PV display frontend")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180508092650.GA661@mwanda