avcodec/mips: Improve avc chroma avg hv mc msa functions

Replace generic with block size specific function.
Load the specific destination bytes instead of MSA load and pack.
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>