Skip av1_find_mv_refs for invalid ref frame pair
Skip av1_find_mv_refs for invalid ref frame pair

1. set_params_rd_pick_inter_mode go through all 21
compound refrence frame combinations, but when any
of the refrence frame is disabled, this pair can be
skipped.

2. For encoder, about 1.0% faster shows by encoding
20 frame of BasketballDrill_832x480_50.y4m.
( 291591 ms -> 288678 ms)

Change-Id: I8a1825935202d91e0387c206fc0936026da54719

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom