drm/virtio: batch display query

Move virtio_gpu_notify() to higher-level functions for
virtio_gpu_cmd_get_display_info() and virtio_gpu_cmd_get_edids().

virtio_gpu_config_changed_work_func() and virtio_gpu_init() will
batch commands and notify only once per update

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20200214125535.26349-6-kraxel@redhat.com