drm/virtio: Remove unused function declarations

Commit dc5698e8 ("Add virtio gpu driver.") declared but never
implemented virtio_gpu_attach_status_page()/virtio_gpu_detach_status_page()
Also commit 62fb7a5e ("virtio-gpu: add 3d/virgl support")
declared but never implemented virtio_gpu_fence_ack() and
virtio_gpu_dequeue_fence_func().
Commit c84adb30

 ("drm/virtio: Support virtgpu exported resources")
declared but never implemented virtgpu_gem_prime_get_uuid().

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230811101823.32344-1-yuehaibing@huawei.com