avcodec/mips: Improve hevc bi 4 tap hz and vt mc msa functions

Use global mask buffer for appropriate mask load.
Signed-off-by: NKaustubh Raste <kaustubh.raste@imgtec.com>
Reviewed-by: NManojkumar Bhosale <Manojkumar.Bhosale@imgtec.com>
Signed-off-by: NMichael Niedermayer <michael@niedermayer.cc>