drm/ast: Managed release of ast firmware

The ast driver loads firmware for the DP501 display encoder. The
patch replaces the removal code with a managed release function.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730135206.30239-8-tzimmermann@suse.de