drm/udl: Use NULL instead of 0 for pointers

Fixes the following sparse warnings:
drivers/gpu/drm/udl/udl_transfer.c:129:50:
drivers/gpu/drm/udl/udl_transfer.c:130:50:
drivers/gpu/drm/udl/udl_transfer.c:131:45:
drivers/gpu/drm/udl/udl_transfer.c:132:61:
warning: Using plain integer as NULL pointer

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>