avcodec/mips: Improve avc avg mc 20, 21 and 23 msa functions

Load the specific destination bytes instead of MSA load and pack.
Remove unused macros 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>