drm/gma500: Move GTT setup and restoration into helper funtions

The GTT init and restore functions contain logic to populate the
GTT entries. Move the code into helper functions.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220308195222.13471-6-tzimmermann@suse.de