drm/i915: clean up virtual PCH special case handling

Use intel_pch_type() also for mapping the no PCH case (PCH id 0) to
PCH_NONE to simplify code.

Also make sure that intel_pch_type() knows all the PCH ids returned by
intel_virt_detect_pch(). Loudly fail if this isn't the case; this
shouldn't happen anyway.

Cc: Colin Xu <Colin.Xu@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Colin Xu <Colin.Xu@intel.com>
Reviewed-by: Colin Xu <Colin.Xu@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180608123330.31003-4-jani.nikula@intel.com

drivers/gpu/drm/i915/i915_drv.c

@@ -284,13 +284,12 @@ static void intel_detect_pch(struct drm_i915_private *dev_priv)
 		} else if (intel_is_virt_pch(id, pch->subsystem_vendor,
 					 pch->subsystem_device)) {
 			id = intel_virt_detect_pch(dev_priv);
-			if (id) {
 			pch_type = intel_pch_type(dev_priv, id);
-				if (WARN_ON(pch_type == PCH_NONE))
-					pch_type = PCH_NOP;
-			} else {
-				pch_type = PCH_NONE;
-			}
+
+			/* Sanity check virtual PCH id */
+			if (WARN_ON(id && pch_type == PCH_NONE))
+				id = 0;
+
 			dev_priv->pch_type = pch_type;
 			dev_priv->pch_id = id;
 			break;