DIX: Set backgroundState correctly for root window

When we change the root window's background to None, and we've run with
-wr or -br for a forced solid background, make sure we also change the
background state to BackgroundPixel, so we don't try to lookup either
pScreen->whitePixel or pScreen->blackPixel as a pixmap.

Signed-off-by: Marko Macek <Marko.Macek@gmx.net>
Reviewed-by: Walter Harms <wharms@bfs.de>
Reviewed-by: Daniel Stone <daniel@fooishbar.org>
Signed-off-by: Keith Packard <keithp@keithp.com>