drm/gma500: Use to_gtt_range() everywhere

Convert upcasts from struct drm_gem_object to struct gtt_range to
to_gtt_range(). Some places used container_of() directly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015084053.13708-3-tzimmermann@suse.de