drm/gma500: Cleanup GTT uninit and error handling

Replace psb_gtt_takedown() with finalizer function that is only called
for unloading the driver. Use roll-back pattern for error handling in
psb_gtt_init() and _resume(). Also fixes a bug where vmap_addr was never
unmapped.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220308195222.13471-8-tzimmermann@suse.de