Inline vp9_get_mv_joint().

Encode time for first 50 frames of bus (speed 0) @ 1500kbps goes from
2min10.9 to 2min10.5, i.e. 0.3% faster overall, basically because we
prevent the call overhead.

Change-Id: I1eab1a95dd3eae282f9b866f1f0b3dcadff073d5