drm/udl: Make udl_enc_destroy() static

Fixes the following sparse warning:
drivers/gpu/drm/udl/udl_encoder.c:19:6: warning:
symbol 'udl_enc_destroy' was not declared. Should it be static?

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