drm/udl: Allocate GEM object via struct drm_driver.gem_create_object

In preparation of a switch to SHMEM, udl now allocates its GEM
objects via struct drm_driver.gem_create_object. No functional
changes are made.

For SHMEM GEM objects, udl will require the use of a special mmap
function, which we set though the create-object function.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107094307.19870-3-tzimmermann@suse.de