drm/qxl: Use drm_fb_helper deferred_io support

Use the fbdev deferred io support in drm_fb_helper which mirrors the
one qxl has had.
This patch has only been compile tested.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1461856717-6476-7-git-send-email-noralf@tronnes.org