drm: Add missing static storage class specifiers in drm_proc.c file

Fixes the following sparse warning:
drivers/gpu/drm/drm_proc.c:92:5:
warning: symbol 'drm_proc_create_files' was not declared. Should it be static?
drivers/gpu/drm/drm_proc.c:175:5:
warning: symbol 'drm_proc_remove_files' was not declared. Should it be static?

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