drm/udl: disable fb_defio by default

There seems to be a bad interaction between gem/shmem and defio on top,
I get list corruption on the page lru in the shmem code.

Turn it off for now until we get some more digging done.

Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>