drm/xen-front: fix pointer casts

Building for a 32-bit target results in warnings from casting
between a 32-bit pointer and a 64-bit integer. Fix the warnings
by casting those pointers to uintptr_t first.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180523113630.29811-1-andr2000@gmail.com