drm/xen: Add missing VM_DONTEXPAND flag in mmap callback

drm/xen: Add missing VM_DONTEXPAND flag in mmap callback

With Xen PV Display driver in use the "expected" VM_DONTEXPAND flag
is not set (neither explicitly nor implicitly), so the driver hits
the code path in drm_gem_mmap_obj() which triggers the WARNING.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: https://lore.kernel.org/r/1652104303-5098-1-git-send-email-olekstysh@gmail.com
Signed-off-by: Juergen Gross <jgross@suse.com>