Only when USE_ALT_REF_ONE_PASS is enabled (off by default).
Force fixed partition to 64x64 when is_src_alt_ref_frame is true,
and don't force early exit for some modes in nonrd_pickmode
for ARF noshow frames.

Small gain ~0.2% on ytlive metrics for speed 6.
Neutral speed difference.

Change-Id: I27eb6622d0453c09a06ccdc3b16368762474d11d