target/riscv: Split pm_enabled into mask and base

target/riscv: Split pm_enabled into mask and base

Use cached cur_pmmask and cur_pmbase to infer the
current PM mode.

This may decrease the TCG IR by one when pm_enabled
is true and pm_base_enabled is false.

Signed-off-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20220120122050.41546-15-zhiwei_liu@c-sky.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>