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/encodetxb.c b/av1/encoder/encodetxb.c
index 0b539b0..c2941a5 100644
--- a/av1/encoder/encodetxb.c
+++ b/av1/encoder/encodetxb.c
@@ -73,8 +73,8 @@
 
 void av1_alloc_txb_buf(AV1_COMP *cpi) {
   AV1_COMMON *cm = &cpi->common;
-  int size = ((cm->mi_rows >> cm->seq_params.mib_size_log2) + 1) *
-             ((cm->mi_cols >> cm->seq_params.mib_size_log2) + 1);
+  int size = ((cm->mi_params.mi_rows >> cm->seq_params.mib_size_log2) + 1) *
+             ((cm->mi_params.mi_cols >> cm->seq_params.mib_size_log2) + 1);
 
   av1_free_txb_buf(cpi);
   // TODO(jingning): This should be further reduced.
@@ -2259,7 +2259,7 @@
                                          int mi_col) {
   const AV1_COMMON *const cm = &cpi->common;
   const int mib_size_log2 = cm->seq_params.mib_size_log2;
-  const int stride = (cm->mi_cols >> mib_size_log2) + 1;
+  const int stride = (cm->mi_params.mi_cols >> mib_size_log2) + 1;
   const int offset =
       (mi_row >> mib_size_log2) * stride + (mi_col >> mib_size_log2);
   return cpi->coeff_buffer_base + offset;