AV1 encoder's logic for handling current_delta_lf_from_base value has
a hole, which could cause enc/dec mismatch due to deblocking filter
level mismatch.

On the encoder side, when delta_lf is used. current_delta_lf_from_base
value is only written into the bit stream when the following condition
is true:

((bsize != BLOCK_LARGEST || skip == 0) && super_block_upper_left)

i.e., it is only written into the bit stream when current mi is the top
left corner of a super block, and it is not skipped or block size is not
64x64. this is applied to both INTRA and INTER blocks in write_mbmi_b().

And the following code is used to handle the case when the above condition
is not true.

if (!dry_run) {
    mbmi = &xd->mi[0]->mbmi;
    if (bsize == BLOCK_64X64 && mbmi->skip == 1 && is_inter_block(mbmi) &&
        cpi->common.delta_lf_present_flag) {
      mbmi->current_delta_lf_from_base = xd->prev_delta_lf_from_base;
    }

The idea is that if the above condition is not true, then the
current_delta_lf_from_base should be reset to the previous delta lf,
which is prev_delta_lf_frame_base.

However, there is a hole in this logic. For INTRA 64x64 blocks, even it
is skipped, the current_delta_lf_frame_base is not reset back to the
previous delta_lf. So, in this case, decoder side won't receive any new
delta_lf from bit stream, so it will use previous delta_lf. encoder side
will use the new delta_lf value. this mismatch will cause final loop
filter level calculation inconsistent in the deblocking stage and thus
cause encoder and decoder mismatch.

BUG=aomedia:826

Change-Id: I90c1dcca5da106e4f3e0bcba1ed539c1cd99c370