drm/i915: Pin cursor bo and unpin old bo when setting cursor.

We also didn't track the cursor bo before and would leak a reference
when the cursor image was change.

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>