drm/gma500: Managed device release

Set up a clean-up action to automatically release device resources
during driver shutdown. Remove manual release code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210920141051.30988-6-tzimmermann@suse.de