Calling drmm_vram_helper_init() sets up a managed instance of
VRAM MM. Releasing the DRM device also frees the memory manager.

The patch also updates the DRM documentation for VRAM helpers. The
tutorial now describes the new managed interface. The old interfaces
are deprecated and should not be used in new code.

v2:
	* rename init function to drmm_vram_helper_init()
	* return errno code from init function; caller does not
	  need vram_mm anyway
	* update documentation and remove docs for deprecated
	  un-managed functions
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716125353.31512-2-tzimmermann@suse.de