VSR: dispatch cbs skipped due to timer lag.

If the timer has a lot of lag relative to the intended wakeup time,
there could be two callbacks that should be dispatched during this
wakeup, not just one, with the additional callback being woken up being
the one that was skipped because of the timer lag.

Fixes: 159884130
Test: 1 new unit test
Test: uibench a/b

Change-Id: Ia17bc5a09a085fcb334226849214e5247e010b37