Currently, trellis optimization is performed in
av1_tx_block_rd_b when var-tx is enabled even when DISABLE_TRELLISQ_SEARCH
is set to 1.

The drop in performance when DISABLE_TRELLISQ_SEARCH is set to 1 is
1.8% on lowres

Change-Id: I89e26d4d4f57944db11b528d0e10048ae650d8a1