drm/gma500: Allocate GTT ranges in stolen memory with psb_gem_create()

Support private objects for stolen memory in psb_gem_create() and
convert users to psb_gem_create(). For stolen memory, psb_gem_create()
now initializes the GEM object via drm_gem_private_object_init().

In the fbdev setup, replace the open-coded initialization of struct
gtt_range with a call to psb_gem_create(). Use drm_gem_object_put()
for release.

In the cursor setup, use psb_gem_create() and get a real GEM object.
Previously the allocated instance of struct gtt_range was only partially
initialized. Release the cursor GEM object in gma_crtc_destroy(). The
release was missing from the original code.

With the conversion of all callers to psb_gem_create(), the extern
declarations of psb_gtt_alloc_range, psb_gtt_free_range and
psb_gem_object_func are not required any longer. Declare them as
static.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015084053.13708-5-tzimmermann@suse.de