drm/vram: Have VRAM MM call GEM VRAM functions directly

VRAM MM and GEM VRAM buffer objects are only used with each other;
connected via 3 function pointers. Simplify this code by making the
memory manager call the rsp. functions of the BOs directly; and
remove the functions from the BO's public interface.

v2:
	* typos in commit message

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190911110910.30698-3-tzimmermann@suse.de