drm/gma500: Remove unused DPST support

DPST never got enabled so remove it. We keep the reg save/restore code
just for safety.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210201132617.1233-2-patrik.r.jakobsson@gmail.com