staging: xgifb: initialize register 3cc address

Commit 56810a92 (staging: xgifb: use
XGIRegInit()) left 3cc uninitialized, and it may trigger a panic during
probe. Fix this.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>