drm/gma500: Inline psb_gtt_{alloc,free}_range() into rsp callers

psb_gtt_alloc_range() allocates struct gtt_range, create the GTT resource
and performs some half-baked initialization. Inline the function into its
only caller psb_gem_create(). For creating the GTT resource, introduce a
new helper, psb_gtt_alloc_resource() that hides the details of the GTT.

For psb_gtt_free_range(), inline the function into its only caller
psb_gem_free_object(). While at it, remove the explicit invocation of
drm_gem_free_mmap_offset(). The mmap offset is already released by
drm_gem_object_release().

v3:
	* replace offset[static 1] with pointer notation (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/20211015084053.13708-8-tzimmermann@suse.de