drm/gma500: Inline psb_gtt_restore()

Inline psb_gtt_restore() into its only caller in power.c.

Perform the GTT restoration in psb_gem_mm_resume(). The restoration
step is part of GEM anyway and will be moved over at some point.

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/20220308195222.13471-10-tzimmermann@suse.de