Both encoder & decoder were using mb_cols to
offset from one row of MODE_INFO structures to the next
when they should have been using mode_info_stride.
Fixing this in both encoder and decoder gives around
a 3KB size saving and 0.025dB PSNR improvement on the one
720P clip I tried.
(Also removed "index" which was being updated but not used)
Change-Id: I413bea802b142886bfcf8d8aa7f5a2f0c524fd4b
ae63ce248a