drm/gma500: Move GTT enable and disable code into helpers

Move the code for enabling and disabling the GTT into helpers and call
the functions in psb_gtt_init(), psb_gtt_fini() and psb_gtt_resume().
Removes code duplication.

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/20220308195222.13471-12-tzimmermann@suse.de