riscv64/itx: Special case 16x16 8bpc dct_dct eob=0

Performance comparison:

- SpacemiT K1:                             Master branch:       itx_16x16:
  inv_txfm_add_16x16_dct_dct_0_8bpc_c:     1534.1 ( 1.00x)      1534.9 ( 1.00x)
  inv_txfm_add_16x16_dct_dct_0_8bpc_rvv:   1173.6 ( 1.31x)       173.1 ( 8.87x)

- Kendryte K230:                           Master branch:       itx_16x16:
  inv_txfm_add_16x16_dct_dct_0_8bpc_c:     1576.0 ( 1.00x)      1579.1 ( 1.00x)
  inv_txfm_add_16x16_dct_dct_0_8bpc_rvv:   1095.5 ( 1.44x)       146.8 (10.75x)