drm/gma500: Rename struct gtt_range to struct psb_gem_object

struct gtt_range represents a GEM object. Rename the structure to struct
psb_gem_object and update all users. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015084053.13708-11-tzimmermann@suse.de