drm/qxl: move shadow handling to new qxl_prepare_shadow()

Pure code motion, no functional change.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210217123213.2199186-10-kraxel@redhat.com