Refactor CommonModeInfoParams from AV1_COMMON

Also, refactor local variables/args for mi_params when we can.

BUG=aomedia:2610

Change-Id: Ifb53494d0e1dd476c2b4a473817ebc583e94a371
diff --git a/av1/encoder/ethread.c b/av1/encoder/ethread.c
index 0ba8d63..3682b68 100644
--- a/av1/encoder/ethread.c
+++ b/av1/encoder/ethread.c
@@ -60,8 +60,8 @@
           av1_reset_loop_filter_delta(xd, av1_num_planes(cm));
         for (int mi_col = tile_info->mi_col_start;
              mi_col < tile_info->mi_col_end; mi_col += mib_size) {
-          const int idx_str = cm->mi_stride * mi_row + mi_col;
-          MB_MODE_INFO **mi = cm->mi_grid_base + idx_str;
+          const int idx_str = cm->mi_params.mi_stride * mi_row + mi_col;
+          MB_MODE_INFO **mi = cm->mi_params.mi_grid_base + idx_str;
           MB_MODE_INFO *mbmi = mi[0];
           if (mbmi->skip == 1 && (mbmi->sb_type == cm->seq_params.sb_size)) {
             for (int lf_id = 0; lf_id < frame_lf_count; ++lf_id)