The commit 3bce7547 has introduced an another early-exit based on MSE distortion
in transform domain, which enables skipping trellis coding and
calling av1_dist_block() in block_rd_txfm() and skipping trellis coding in av1_tx_block_rd_b().

However, with dist-8x8, the early-exit for sub8x8 tx block in a partition >= 8x8 in plane 0
is disabled because that the reference distortion metric
(which would be non-MSE and applied to 8x8 or larger) can not be compared to
MSE distortions of sub8x8 tx blocks.

Change-Id: I46ada7c90a869d23fc0f0166a01dfdc5392af311