For real-time mode. Move the switch to fixed partition
for is_src_frame_alt_ref so all speeds may use it
if use_altref_onepass is set.

Improves metrics by ~2% for ytlive set at speed 4
(where use_altref_onepass is currently used).

Change-Id: I033240386598c9dbd0364da89ccbcca64bc663ee