OpenBIOS on sparc64 only support Standard VGA and not Cirrus VGA. Don't
build Cirrus VGA support so that it can't be selected.

This fixes the breakage introduced by commit f2898771.

Reported-by: Richard Henderson <rth@twiddle.net>
Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Tested-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>