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: NThomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190508082630.15116-3-tzimmermann@suse.deSigned-off-by: NGerd Hoffmann <kraxel@redhat.com>