drm/udl: properly check for error pointers

The drm_prime_pages_to_sg() function never returns NULL pointers, only
error pointers and valid pointers.

Signed-off-by: Haixia Shi <hshi@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>