drm: unexport drm_global_mutex

Drivers really, really have no business even looking at this lock. And
thankfully they don't.

So unexport it and move the declaration to drm_internal.h.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_fops.c

@@ -43,7 +43,6 @@
 
 /* from BKL pushdown */
 DEFINE_MUTEX(drm_global_mutex);
-EXPORT_SYMBOL(drm_global_mutex);
 
 static int drm_open_helper(struct file *filp, struct drm_minor *minor);