Fix in the case of MMCO 3 (long term reference idx).

Increment number of long term reference buffers only when both top field
and bottom field have been set as long term.

[backport for M/N from master]

Bug: 35584425
Test: ran POC - no hang, no segfault.

Change-Id: I94e3857944da675eda38f8e1a9bd887f48bff524
(cherry picked from commit 6fa5df8811ea0b8e8459f86dd3c30bf7a9b39482)
(cherry picked from commit 46e96d40dbca2896b5e20cf48d14798231c97663)