This is similar to h264, but here we use manual_avg instead of vaaddu
because rv40's OP differs from h264. If we use vaaddu,
rv40 would need to repeatedly switch between vxrm=0 and vxrm=2,
and switching vxrm is very slow.

C908:
avg_chroma_mc4_c: 2330.0
avg_chroma_mc4_rvv_i32: 602.7
avg_chroma_mc8_c: 1211.0
avg_chroma_mc8_rvv_i32: 602.7
put_chroma_mc4_c: 1825.0
put_chroma_mc4_rvv_i32: 414.7
put_chroma_mc8_c: 932.0
put_chroma_mc8_rvv_i32: 414.7
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>