sched/tune: don't use schedtune before it is ready

When EAS is enabled during boot, we have to be careful not to use
schedtune from fair.c before it is ready or it will warn us and we'll
get a traceback in the console.

Change-Id: I1a5cf29b18af626545c636c51219f9ed497c19fa
Signed-off-by: Chris Redpath <chris.redpath@arm.com>