drm/i915: clarify IBX dp workaround

Instead of checking for !CPT, check for IBX to make it clearer that
this is a IBX-specific workaround. No functional change because we
smash the PPT PCH into the HAS_PCH_CPT check and atm DP isn't enabled
on the haswell LPT PCH yet.

See Bspec Vol 3, Part 3, Section 4.[3-5].1 about DP[BCD], bit 30 for
details of this workaround.
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dp.c

@@ -1914,7 +1914,7 @@ intel_dp_link_down(struct intel_dp *intel_dp)
 			DP |= DP_LINK_TRAIN_OFF;
 	}
 
-	if (!HAS_PCH_CPT(dev) &&
+	if (HAS_PCH_IBX(dev) &&
 	    I915_READ(intel_dp->output_reg) & DP_PIPEB_SELECT) {
 		struct drm_crtc *crtc = intel_dp->base.base.crtc;