Powering HVX on/off is racy

[dsharletg]

There are potential race conditions in halide_hexagon_power_hvx_on and halide_hexagon_power_hvx_off. There are at least three possible issues:

1. The reference count is not atomically incremented/decremented (fixed in Fix potential race condition in power_hvx_on/off #5194). This is the worst one, because it could result in HVX not being turned on or off at all when needed.
2. HVX could be powered off, and while it is powering off, be powered back on. I'm not sure what would happen in this case or which call would "win".
3. After Fix potential race condition in power_hvx_on/off #5194, it is possible that two calls to power_hxv_on could both attempt to power it on. Hopefully, the Hexagon system software can handle this case.