drm/vbox: Remove empty PRIME functions

The vbox driver uses VRAM helpers for memory management. These helpers
provide a basic implementation of PRIME functions, so the vbox driver's
empty implmentation can be removed. As a side effect of this change,
vbox can now use generic framebuffer emulation.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190702115012.4418-5-tzimmermann@suse.de