drivers: intc: plic: refactor lock in `plic_irq_enable_set_state()`

Move the lock out from the `plic_irq_enable_set_state()` function
to cover the entire configuration process, so the whole of
enable/disable is atomic.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>