virtio-gpu: Support asynchronous fencing

Support asynchronous fencing feature of virglrenderer. It allows Qemu to
handle fence as soon as it's signalled instead of periodically polling
the fence status. This feature is required for enabling DRM context
support in Qemu because legacy fencing mode isn't supported for DRM
contexts in virglrenderer.

Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Yiwei Zhang <zzyiwei@gmail.com>
Tested-by: Yiwei Zhang <zzyiwei@gmail.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Message-ID: <20260303151422.977399-10-dmitry.osipenko@collabora.com>
Message-ID: <20260304165043.1437519-12-alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>