Fix another multithreaded encoder loopfilter race condition

After a key frame encoding, the frame type could change while
filtering is still going on. Pass the frame type as parameter to the
loopfilter function and don't read it from common storage.

vp8cx_set_alt_lf_level has to be done before packing the stream.
Currently alt_lf_level is not used so there hasn't been any visible
problem here.

Change-Id: Ia114162158cd833c2b16e3b89303cc9c91f19165