avcodec/mips: Improve hevc bi copy mc msa functions

Load the specific destination bytes instead of MSA load and pack.
Use immediate unsigned saturation for clip to max saving one vector register.
Signed-off-by: Kaustubh Raste <kaustubh.raste@imgtec.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>