sf: Construct Phase Offsets for unknown fps

External display can support a Refresh Rate value which doesn't exist
in keys of mOffsets map which has been created with RefreshRateConfigs
of Primary display, based on current design. This is not a fatal error
scenario. In such a case, construct Phase Offsets with current logic.

CRs-Fixed: 2643508
Bug: 151859083
Test: boot
Change-Id: Ib89f0dbc3db6bdd3e41681c31c567890487c453f