Fix the bug in coeff context indexing for chroma planes
Fix the bug in coeff context indexing for chroma planes

With level map coding, the coefficient contexts from above and left
can be more than one element which is all the adjacent contexts to the
current tx block size.
(Prior to level map, only one left and above contexts are used to choose a cdf)

Fix is to apply the same indexing method as used in set_coeff_context().

The test to confirm the bug and fix is to enforce all first SB row
and firt SB column to use 4x4 txs, so that it can be contexts for next rows and columnms
which I enforce to use 16x16 tx blockss or 32x32 tx blocks.

xiph/rav1e,xiph/rav1e