target/riscv: Reorg access check in get_physical_address

target/riscv: Reorg access check in get_physical_address

We were effectively computing the protection bits twice,
once while performing access checks and once while returning
the valid bits to the caller.  Reorg so we do this once.

Move the computation of mxr close to its single use.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Tested-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-Id: <20230325105429.1142530-25-richard.henderson@linaro.org>
Message-Id: <20230412114333.118895-25-richard.henderson@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>