drm/gma500: Move GTT resume logic out of psb_gtt_init()

The current implementation of psb_gtt_init() also does resume
handling. Move the resume code into its own helper.

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/20220308195222.13471-7-tzimmermann@suse.de