staging: xgifb: validate the mode against video memory size

It's possible to select video mode that exceeds the available video
memory. This is potentially dangerous, fix by adding a check.

The patch fixes system hangs seen occasionally when playing random videos
with mplayer.

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