systemc: avoid mutex lock in non async cases

systemc: avoid mutex lock in non async cases

Avoid acquiring a mutex lock in case there is no async update in the
scheduler. This helps increasing simulation speed by about 4%.

Change-Id: I971c7bf1a1eeb46208eeee6e5da6385c907092b3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34695
Reviewed-by: Earl Ou <shunhsingou@google.com>
Maintainer: Earl Ou <shunhsingou@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>