SF: turn off kernel idle timer when refresh rate switching is disabled

Previously we had an optimization to keep the timer state as is if
the lowest refresh rate setting is the refresh rate the device is locked
at and refresh rate switching is disabled. Removing this optimization
as it is confusing and doesn't provide a lot of added value.

Bug: 191435579
Test: SF unit tests
Change-Id: I206d625c2fafd27623e2c1353a48d56a3b771c7b