drm/i915: Rename unpin_count to pin_count

We increase it when we pin, so for the casual reader
rename it to cause less confusion.

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Thomas Daniel <thomas.daniel@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>