drm/vram: Unexport internal functions of VRAM MM

The init, cleanup and mmap functions of VRAM MM are only used internally.
Remove them from the public interface.

v2:
	* update for debugfs support

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190911110910.30698-4-tzimmermann@suse.de