avcodec/mips: Improve avc avg mc 10, 30, 01 and 03 msa functions

Align the mask buffer to 64 bytes.
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>