virtio-gpu: Replace finish_unmapping with mapping_state

Allow virtio_gpu_virgl_unmap_resource_blob() to be invoked while async
unmapping is in progress. Do it in preparation to improvement of virtio-gpu
resetting that will require this change.

Suggested-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Message-ID: <20260303151422.977399-17-dmitry.osipenko@collabora.com>
Message-ID: <20260304165043.1437519-19-alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>