target-sparc64: disable VGA cirrus

target-sparc64: disable VGA cirrus

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>
(cherry picked from commit 0356404b0f1da939657cad1efeb556745cd430d5)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>