drm: Add |struct drm_gem_vram_object| callbacks for |struct ttm_bo_driver|

The provided helpers can be used for the respective callback functions
in |struct ttm_bo_driver|.

v2:
	* drm_is_gem_vram() is now a private function
	* documentation fixes

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190508082630.15116-3-tzimmermann@suse.de


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