drm/virtio: batch display update commands.

When the driver submits multiple commands in a row it makes sense to
notify the host only after submitting the last one, so the host can
process them all at once, with a single vmexit.

Add functions to enable/disable notifications to allow that.  Use the
new functions for primary plane updates.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20191212125346.8334-3-kraxel@redhat.com