drm/ast: Managed device release

This turns the ast's device cleanup code into a managed release helper
function. Note that the code uses devres helpers. The release function
switches the device back to VGA mode and therefore runs during HW device
cleanup; not at DRM device cleanup.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730135206.30239-10-tzimmermann@suse.de