SoftAvc: Fix input offset reset during port reconfiguration

During port reconfiguration, input buffer offset should not be
set to zero. In case input contains more than one header and second
header triggers a change in resolution, then input offset should point to
second header in the subsequent calls to decoder.

Bug: 67761639
Test: ran PoC before/after
Change-Id: I710653506959752d28170e656c1a5a897b02b335