The partition_info struct contains info just for SPLITMV,
so it should be used instead of BLOCKD.  Eventually, I want
to reduce the size of B_MODE_INFO struct found in BLOCKD, so
this is the first step toward that goal.
Also, since SPLITMV is not supported in vp8_pick_inter_mode(),
the unnecessary mem copies and checks were removed.  For rt
encodes, this gave a slight performance improvement.

Change-Id: I5585c98fa9d5acbde1c7e0f452a01d9ecc080574