drm/gma500/psb: Fix cursor gem obj referencing on psb

The internal crtc cursor gem object pointer was never set/updated since
it was required to be set in the first place.

Fixing this will make the pin/unpin count match and prevent cursor
objects from leaking when userspace drops all references to it. Also
make sure we drop the gem obj reference on failure.

This patch only affects Poulsbo chips.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>