FB: sa1100: make GPIO configuration setting safe

The sa1100fb driver needs to set the GPIO direction and alternate
function register according to the panel that we're driving.  We've
done this in the driver by read-modify-writing the register, which
may cause problems with races.  Fix this with a minimal change.

Acked-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>