CompositionEngine: only offload when displays are enabled

When using multithreaded_present, only offload any displays if there are
more than one enabled display. If e.g. there are two displays, but one
is not enabled, there is not the same benefit of offloading it.

Bug: 241285491
Bug: 259132483
Test: libcompositionengine_test
Change-Id: Iad4a6fc47127b5ab945108f8c6ede1216755adbd