drm: Add default instance for VRAM MM callback functions

VRAM MM is most likely be used with GEM VRAM. The latter now provides the
required instance of struct drm_vram_mm_funcs for drivers to use.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190508082630.15116-8-tzimmermann@suse.de


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>