SF: remove DispSyncSource

DispSyncSource is a legacy reminder of DispSync which was removed
a while ago. This changes removes the DispSyncSource and instead use
the VsyncDispatch interface directly.

Test: SF unit tests
Bug: 255601557
Bug: 256196556
Change-Id: I4247c3927b9ee9443437bf960f8cedc29759c4ac