drm/i915/selftest: Add throttle reason diagnostics to RPS selftests

drm/i915/selftest: Add throttle reason diagnostics to RPS selftests

Report GPU throttle reasons when RPS tests fail to reach expected
frequencies or power levels.

v2: Read the throttle value before the spinner ends (Raag)
    Add a condition before printing throttle value (Krzysztof)
v3: Extend throttle reasons debug support (Raag)
v4: Cosmetic changes (Raag)

Signed-off-by: Sk Anirban <sk.anirban@intel.com>
Reviewed-by: Raag Jadav <raag.jadav@intel.com>
Reviewed-by: Krzysztof Karas <krzysztof.karas@intel.com>
Link: https://patch.msgid.link/20251223101605.3304379-2-sk.anirban@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>