avcodec/mips: Improve hevc uni weighted 4 tap vt mc msa functions

Use global mask buffer for appropriate mask load.
Use immediate unsigned saturation for clip to max saving one vector register.
Remove unused macro.
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>