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

Current performance comparison:

SpacemiT K1:	Master branch:	itx_16x16:
inv_txfm_add_16x16_dct_dct_0_8bpc_c:	1526.8 ( 1.00x)	1533.2 ( 1.00x)
inv_txfm_add_16x16_dct_dct_0_8bpc_rvv:	1151.4 ( 1.33x)	176.8 ( 8.67x)

Kendryte K230:	Master branch:	itx_16x16:
inv_txfm_add_16x16_dct_dct_0_8bpc_c:	1731.3 ( 1.00x)	1730.7 ( 1.00x)
inv_txfm_add_16x16_dct_dct_0_8bpc_rvv:	1100.5 ( 1.57x)	153.5 (11.28x)