Use hwmon to determine which sysfs path to use for gpu/power bits.

Instead of always using /sys/class/drm/card0 based paths, use
/sys/class/hwmon looking for amdgpu name file to determine where
to look.
Fixes reading TDPLimit, GPUPerformanceLevel and ManualGPUClock
in cases where sysfs uses card1 or other paths.
NOTE: Using wrong cardX path is a bug and will be fixed, but
this makes steamos-manager a bit more resilient to those bugs.

src/manager.rs

@@ -445,6 +445,7 @@ mod test {
 
         let proxy = ManualGpuClockProxy::new(&test.connection).await.unwrap();
 
+        power::test::setup().await;
         assert!(proxy.manual_gpu_clock().await.is_err());
 
         power::test::write_clocks(1600).await;