drm/ast: Use managed MM initialization

Cleaning up ast's MM code with ast_mm_fini() resets the write-combine
flags on the VRAM I/O memory. Drop ast_mm_fini() in favor of an auto-
release callback. Releasing the device also executes the callback.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716125353.31512-7-tzimmermann@suse.de