drm/i915: fix pch_nop support

This was accidentally broken in the south error interrupt handling
work:

commit 8664281b64c457705db72fc60143d03827e75ca9
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Fri Apr 12 17:57:57 2013 -0300

    drm/i915: report Gen5+ CPU and PCH FIFO underruns

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -2551,6 +2551,9 @@ static void ibx_irq_postinstall(struct drm_device *dev)
 	drm_i915_private_t *dev_priv = (drm_i915_private_t *) dev->dev_private;
 	u32 mask;
 
+	if (HAS_PCH_NOP(dev))
+		return;
+
 	if (HAS_PCH_IBX(dev)) {
 		mask = SDE_GMBUS | SDE_AUX_MASK | SDE_TRANSB_FIFO_UNDER |
 		       SDE_TRANSA_FIFO_UNDER | SDE_POISON;
@@ -2560,9 +2563,6 @@ static void ibx_irq_postinstall(struct drm_device *dev)
 		I915_WRITE(SERR_INT, I915_READ(SERR_INT));
 	}
 
-	if (HAS_PCH_NOP(dev))
-		return;
-
 	I915_WRITE(SDEIIR, I915_READ(SDEIIR));
 	I915_WRITE(SDEIMR, ~mask);
 }