drm/gma500: Move GTT memory-range setup into helper

Move the setup code for GTT/GATT memory ranges into a new helper and
call the function from psb_gtt_init() 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/20220308195222.13471-13-tzimmermann@suse.de