drm/vboxvideo: Replace prepare_fb()/cleanup_fb() with GEM VRAM helpers

GEM VRAM provides an implementation for prepare_fb() and cleanup_fb()
of struct drm_plane_helper_funcs. Switch over vboxvideo.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191024081404.6978-5-tzimmermann@suse.de