drm: Add misssing static storage class specifier in drm_fb_helper.c file

Fixes the following sparse warning:
drivers/gpu/drm/drm_fb_helper.c:239:6: warning:
symbol 'drm_fb_helper_force_kernel_mode' was not declared. Should it be static?

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