drm/xen: drop use of drmP.h

The drmP.h header is deprecated.  Drop all uses.
Added includes/forwards to the header files and
then fixed fallout in the .c files.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Cc: xen-devel@lists.xenproject.org
Link: https://patchwork.freedesktop.org/patch/msgid/20190630061922.7254-3-sam@ravnborg.org