drm/i915: properly reenable gen8 pipe IRQs

We were missing the pipe B/C vblank bits! Take a look at
gen8_de_irq_postinstall for a comparison.

This should fix a bunch of IGT tests.

There are a few more things we could improve on this code, but this
should be the minimal fix to unblock us.

v2: s/extra_iir/extra_ier/ because IIR doesn't make sense (Ville)

Bugzilla:https://bugs.freedesktop.org/show_bug.cgi?id=83640
Testcase: igt/*
Cc: stable@vger.kernel.org
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_irq.c

@@ -3458,12 +3458,13 @@ static void gen8_irq_reset(struct drm_device *dev)
 void gen8_irq_power_well_post_enable(struct drm_i915_private *dev_priv)
 {
 	unsigned long irqflags;
+	uint32_t extra_ier = GEN8_PIPE_VBLANK | GEN8_PIPE_FIFO_UNDERRUN;
 
 	spin_lock_irqsave(&dev_priv->irq_lock, irqflags);
 	GEN8_IRQ_INIT_NDX(DE_PIPE, PIPE_B, dev_priv->de_irq_mask[PIPE_B],
-			  ~dev_priv->de_irq_mask[PIPE_B]);
+			  ~dev_priv->de_irq_mask[PIPE_B] | extra_ier);
 	GEN8_IRQ_INIT_NDX(DE_PIPE, PIPE_C, dev_priv->de_irq_mask[PIPE_C],
-			  ~dev_priv->de_irq_mask[PIPE_C]);
+			  ~dev_priv->de_irq_mask[PIPE_C] | extra_ier);
 	spin_unlock_irqrestore(&dev_priv->irq_lock, irqflags);
 }