drm/vboxvideo: Convert vboxvideo driver to |struct drm_gem_vram_object|

drm/vboxvideo: Convert vboxvideo driver to |struct drm_gem_vram_object|

This patch replaces |struct vbox_bo| and its helpers with the generic
implementation of |struct drm_gem_vram_object|. The only change in
semantics is that &ttm_bo_driver.verify_access() now does the actual
verification.

v4:
	* select config option DRM_VRAM_HELPER
v3:
	* remove forward declaration of struct vbox_gem_object
v2:
	nothing

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190508082630.15116-18-tzimmermann@suse.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>