The power resource dependencies for each `_PRx` object are discovered and cached in `ac_prx` on the power consumer struct (`struct acpi_powerconsumer`) when a power consumer is registered. This is done in `acpi_pwr_get_power_resources`. ACPI guarantees these `_PRx` objects will evaluate to the same thing each time they are called.
This discovery process also registers those power resources, which were previously only registered when they were referenced by the relevant `_PRx` object for the target D-state when switching. This meant that the first D-state switch for a power consumer would not turn off any power resources as they wouldn't have been registered yet. This revision fixes this.
`ac_prx` will be used by subsequent patches.
Sponsored by: The FreeBSD Fondation