radv: Add `RADV_PERF_PSTATE` envvar

When profiling on my hawk point laptop, it's not possible to sustain profile_peak clocks without overheating, even when manually forcing the fan to maximum. Without manually forcing the fan to max, the system will overheat and shutdown in a matter of seconds, which is unfortunate.

This MR allows one to select the specific pstate to enter when profiling, so I can use profile_standard instead of profile_peak and keep my laptop from cooking itself. I'm not really sure how useful the other options are to people, but since I was there I just banged out something which covers all the options supported by amdgpu. Note it's not possible to choose anything other than a profile_ state, since that is required for functionality.

(I wasn't really sure whether I should bother caching the chosen mode, so I just did it the lazy way)

radv: Add `RADV_PERF_PSTATE` envvar

Enable selecting the specific pstate to enter when using thread tracing
and when acquiring the profiling lock for performance queries.

Signed-off-by: Josh Simmons <josh@nega.tv>