drm/gma500: Move helpers for struct gtt_range from gtt.c to gem.c

Allocation and pinning helpers for struct gtt_range are GEM functions,
so move them to gem.c. No functional changes.

v2:
	* keep docs for psb_gtt_{attach,detach}_pages() (Patrik)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015084053.13708-2-tzimmermann@suse.de