SurfaceFlinger: Dispsync changePhaseOffset

When changing the phase offset of DispSync there is no need
to adjust the last event time beyond the diff between the old
offset and the new one.

Test: default <-> early offset transition
Test: default <-> earlyGL offset transition
Bug: 135215384
Change-Id: I436c63914e53ddb53963c590493ee160efb62db8