drm/ast: Use managed VRAM-helper initialization

As a first step to managed MM code in ast, switch over VRAM MM helpers.

v2:
	* updated to use drmm_vram_helper_init()

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716125353.31512-4-tzimmermann@suse.de