drm/gma500: Acquire reservation lock for GEM objects

Protect concurrent access to struct psb_gem_object by acquiring
the GEM object's reservation lock; as it's supposed to be. The
use of the GTT mutex can now be moved into GTT code.

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/20220308195222.13471-3-tzimmermann@suse.de