systemc: Use the eq pointer directly less often in the scheduler.

systemc: Use the eq pointer directly less often in the scheduler.

This takes advantage of the utility functions that exist a little
better, and also avoids accidentally asymetrically using
eventsToSchedule and eq->(de)schedule.

Change-Id: I1eb1c228d47684cccb9deaf6f3409b77cfbad4cd
Reviewed-on: https://gem5-review.googlesource.com/12260
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>