avcodec/mips: Improve avc avg mc 02, 12 and 32 msa functions

Remove loops and unroll as block sizes are known.
Load the specific destination bytes instead of MSA load and pack.
Remove unused macro and functions.
Signed-off-by: Kaustubh Raste <kaustubh.raste@imgtec.com>
Reviewed-by: Manojkumar Bhosale <Manojkumar.Bhosale@imgtec.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>