drm/i915: Drop pointless WARN_ON

intel_dp_link_down() is static and it's only called from the pre-ddi
DP functions, so having a WARN_ON(HAS_DDI) in there is quite pointless.
Remove it.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190604140214.9947-2-ville.syrjala@linux.intel.comReviewed-by: Manasi Navare <manasi.d.navare@intel.com>

drivers/gpu/drm/i915/intel_dp.c

@@ -3995,9 +3995,6 @@ intel_dp_link_down(struct intel_encoder *encoder,
 	enum port port = encoder->port;
 	u32 DP = intel_dp->DP;
 
-	if (WARN_ON(HAS_DDI(dev_priv)))
-		return;
-
 	if (WARN_ON((I915_READ(intel_dp->output_reg) & DP_PORT_EN) == 0))
 		return;