This patch fixes up av1_tx_type_cost to match the code in
av1_write_tx_type. Beforehand, we wrongly assumed a 32x16 block needed
to signal its transform size (with rect-tx-ext & rect-tx-ext-intra)
because we were passing 16x16 to get_ext_tx_types.

I've also changed av1_write_tx_type to use get_min_tx_size rather than
inlining its body. No functional change, but it's probably better to
use the same helper function both times.

Change-Id: Iff6ee0bff2d332d5270fe0219db88c95e0b051d0