ARM: footbridge: make screen_info setup conditional

The global screen_info structure is used to communicate
data about the console from platform code to the console
driver, but is only defined on ARM if either the VGA or
dummy consoles are in use.

This changes the footbridge code so we don't try to access
this structure in case it is not defined, which prevents
a possible randconfig build error.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@arm.linux.org.uk>