drm/virtio: add trace events for commands

Trace when commands are queued for both ctrlq and cursorq. Trace
when responses are received for ctrlq.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190429220825.156644-3-olvaffe@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>