xen: flag xen_drm_front to be not essential for system boot

Similar to the virtual frame buffer (vfb) the pv display driver is not
essential for booting the system. Set the respective flag.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20211022064800.14978-3-jgross@suse.com


Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>