drm/i915: don't write powered down IRQ registers on Gen 8

If we enable unclaimed register reporting on Gen 8, we will discover
that the IRQ registers for pipes B and C are also on the power well,
so writes to them when the power well is disabled result in unclaimed
register errors.

Also, hsw_power_well_post_enable() already takes care of re-enabling
them once the power well is enabled.

Testcase: igt/pm_rpm/rte
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -3467,6 +3467,8 @@ static void gen8_irq_reset(struct drm_device *dev)
 	gen8_gt_irq_reset(dev_priv);
 
 	for_each_pipe(pipe)
+		if (intel_display_power_enabled(dev_priv,
+						POWER_DOMAIN_PIPE(pipe)))
 			GEN8_IRQ_RESET_NDX(DE_PIPE, pipe);
 
 	GEN5_IRQ_RESET(GEN8_DE_PORT_);
@@ -3800,7 +3802,10 @@ static void gen8_de_irq_postinstall(struct drm_i915_private *dev_priv)
 	dev_priv->de_irq_mask[PIPE_C] = ~de_pipe_masked;
 
 	for_each_pipe(pipe)
-		GEN8_IRQ_INIT_NDX(DE_PIPE, pipe, dev_priv->de_irq_mask[pipe],
+		if (intel_display_power_enabled(dev_priv,
+				POWER_DOMAIN_PIPE(pipe)))
+			GEN8_IRQ_INIT_NDX(DE_PIPE, pipe,
+					  dev_priv->de_irq_mask[pipe],
 					  de_pipe_enables);
 
 	GEN5_IRQ_INIT(GEN8_DE_PORT_, ~GEN8_AUX_CHANNEL_A, GEN8_AUX_CHANNEL_A);