drm/vboxvideo: Replace struct vram_framebuffer with generic implemenation

The vboxvideo driver's struct vram_framebuffer stores a DRM framebuffer
with an assiciated GEM object. This functionality is also provided by
generic code. Switch vboxvideo over.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191011134808.3955-4-tzimmermann@suse.de