correct zbin boost for splitmv mode

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

vp8/encoder/rdopt.c

@@ -1968,7 +1968,7 @@ int vp8_rd_pick_inter_mode(VP8_COMP *cpi, MACROBLOCK *x, int recon_yoffset, int
                     else
                         cpi->zbin_mode_boost = LF_ZEROMV_ZBIN_BOOST;
                 }
-                else if (vp8_ref_frame_order[mode_index] == SPLITMV)
+                else if (vp8_mode_order[mode_index] == SPLITMV)
                     cpi->zbin_mode_boost = 0;
                 else
                     cpi->zbin_mode_boost = MV_ZBIN_BOOST;