drm/qxl: add drm_driver.release callback.

Move final cleanups to qxl_drm_release() callback.
Add drm_atomic_helper_shutdown() call to qxl_pci_remove().

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20200210113753.5614-3-kraxel@redhat.com