cpu-o3: Prevent cpu from suspending if it is already draining

cpu-o3: Prevent cpu from suspending if it is already draining

Suspending the current thread context while draining due to a quiesce
pseudo instruction (for example a wfi instruction) could deadlock the
cpu and prevent it from successfully draining. This change ensures
that the cpu is not draining before suspending the thread context.

Change-Id: I7c019847f5a870d4bc9ce2b19936bc3dc45e5fd7
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5881
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>