Simplifies decoder multithread synching

Increment the last_row_mb_col counter by nsync after last MB of row is
ready. This way we dont need to check for last MB of row when
synching.

Set last MB of row ready just after row extension is done, This avoids
o potential race condition whit the processing of last MB of next row.

Change-Id: I19c44fd6041116ee5483be2143b4f4bfcd149eac