drm/ast: Set up release action right after enabling MMIO

Ast sets up a managed release of the MMIO access flags. Move this
code next to the MMIO access code, so that it runs if other errors
occur during the device initialization.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Sui Jingfeng <suijingfeng@loongson.cn> # AST2400
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com> # AST2600
Link: https://patchwork.freedesktop.org/patch/msgid/20230621130032.3568-9-tzimmermann@suse.de