Disable zbin boost in SPLITMV mode as intended. Was incorrectly looking
at vp8_ref_frame_order instead of vp8_mode_order when comparing against
SPLITMV. This condition should have always been false, as SPLITMV is
not in the range of valid reference frames.

Change-Id: I0408cc7595eff68f00efef6d008e79f5b60d14bf