target/riscv: Fix initialized value for cur_pmmask

target/riscv: Fix initialized value for cur_pmmask

We initialize cur_pmmask as -1(UINT32_MAX/UINT64_MAX) and regard it
as if pointer mask is disabled in current implementation. However,
the addresses for vector load/store will be adjusted to zero in this
case and -1(UINT32_MAX/UINT64_MAX) is valid value for pmmask when
pointer mask is enabled.

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-Id: <20230610094651.43786-1-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>