udl-kms: fix crash due to uninitialized memory

We must use kzalloc when allocating the fb_deferred_io structure.
Otherwise, the field first_io is undefined and it causes a crash.

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