drm/xen: Simplify fb_create

The current code is a pretty good wtf moment, since we drop the
reference before we use it. It's not a big deal, because a) we only
use the pointer, so doesn't blow up and the real reason b) fb->obj[0]
already holds a full reference for us.

Might as well take the real pointer ins't of complicated games that
baffle.

Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Cc: xen-devel@lists.xenproject.org
Link: https://patchwork.freedesktop.org/patch/msgid/20191115092120.4445-7-daniel.vetter@ffwll.ch