drm/virtio: add drm_driver.release callback.

Split virtio_gpu_deinit(), move the drm shutdown and release to
virtio_gpu_release().  Drop vqs_ready variable, instead use
drm_dev_{enter,exit,unplug} to avoid touching hardware after
device removal.  Tidy up here and there.

v4: add changelog.
v3: use drm_dev_*().

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20200211135805.24436-1-kraxel@redhat.com