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>

drivers/gpu/drm/drm_fb_helper.c

@@ -236,7 +236,7 @@ bool drm_fb_helper_restore_fbdev_mode(struct drm_fb_helper *fb_helper)
 }
 EXPORT_SYMBOL(drm_fb_helper_restore_fbdev_mode);
 
-bool drm_fb_helper_force_kernel_mode(void)
+static bool drm_fb_helper_force_kernel_mode(void)
 {
 	bool ret, error = false;
 	struct drm_fb_helper *helper;