Commit https://aomedia-review.googlesource.com/c/aom/+/39985
changed the deblock filter for subsampled planes, so that
we always look at the bottom-right mi unit in each 8x8 luma
pixel unit. This fixed some issues around determining which
blocks are inter/skip/etc.

However, this introduced a different bug - the boundary info
("are we allowed to filter across the top/left edge of this block?")
is stored in the *top-left* mi unit in each 8x8 luma pixel unit.
But now we we're looking at the bottom-right unit, which always says
it's fine to filter across the top and left boundaries! Thus the
deblock filter effectively ignored the
loop_filter_across_tiles_enabled flag(s).

Fix this issue by looking up the boundary info from the top-left
block in each unit; the other information is still taken from
the bottom-right.

Change-Id: I626ea3358563641a2532ee9c5968fb54bdc78e34