fix a bug related to gf_active_flags in multi-threaded encoder

Paul pointed out that the pointer to the gf_active_flags is not being
properly incremented in multithreaded encoder. This commit fixes the
issue by making sure the gf_active_ptr points to the starting of next
group of mb rows.

Change-Id: I3246e657d23beabb614dfb880733a68a5fd7e34c