Commit ea190906 fixed a major bug in has_top_right(), but missed
out one special case:

Consider a 128x128 block using 64x64 intra predictions. These
intra predictions are applied in a 'Z' order, and so the bottom-left
64x64 unit has pixels available from the top-right one. But we were
mistakenly setting has_top_right() = 0 for this case.

More generally, whichever transform unit has its top-right corner
at the center of a 128x128 block, should have has_top_right() = 1.

Fix this by introducing an explicit check for the special case.

BUG=aomedia:1309

Change-Id: I690a292be6c1755c76bd428be94ab953dd71fbd2