GPU performance state should be an integer

[lars-t-hansen]

Due to working with NVIDIA first and later only adding other GPUs, the perf state is a string. But it should be an integer >= 0 for a known state (card-specific) and -1 for unknown. This is how the cards report it anyway. This will slightly change the output format but maybe that's OK, I think they were introduced in 0.13 which remains unreleased.