drm/udl: Remove struct udl_gem_object and functions

Simply removes all the obsolete GEM code from udl. No functional
changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107094307.19870-5-tzimmermann@suse.de