drm/i915: tune down the noise of the RP irq limit fail

We still don't understand why this fails exactly, but if fails way
too often for a simple debug information. Furthermore the current
ducttape should prevent the gpu from getting stuck at low frequencies.

Hence tune down the dmesg noise.

Note that the known failure case is that the register read returns 0
when the gpu gets confused.

v2: Add comments about the known failure case.
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -3603,8 +3603,9 @@ static void gen6_sanitize_pm(struct drm_device *dev)
 		limits |= (dev_priv->min_delay & 0x3f) << 16;
 
 	if (old != limits) {
-		DRM_ERROR("Power management discrepancy: GEN6_RP_INTERRUPT_LIMITS expected %08x, was %08x\n",
-			  limits, old);
+		/* Note that the known failure case is to read back 0. */
+		DRM_DEBUG_DRIVER("Power management discrepancy: GEN6_RP_INTERRUPT_LIMITS "
+				 "expected %08x, was %08x\n", limits, old);
 		I915_WRITE(GEN6_RP_INTERRUPT_LIMITS, limits);
 	}