drm/udl: Add missing static storage class specifiers in udl_fb.c

Fixes the following sparse warnings:
drivers/gpu/drm/udl/udl_fb.c:360:6: warning:
symbol 'udl_crtc_fb_gamma_set' was not declared. Should it be static?
drivers/gpu/drm/udl/udl_fb.c:365:6: warning:
symbol 'udl_crtc_fb_gamma_get' was not declared. Should it be static?

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