drm/i915/skl: Restore the DDI translation tables when enabling PW1

I was dumping the DDI translation tables to make sure my patch updating
the HDMI entry was doing the right thing when I noticed that the table
was showing reset values after DPMS.

And indeed, the DDI translation registers are in power well 1 on SKL,
and so we're losing their values when shutting down eDP.

Calling intel_prepare_ddi() on PW1 enabling re-programs the table.
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_runtime_pm.c

@@ -223,8 +223,10 @@ static void skl_power_well_post_enable(struct drm_i915_private *dev_priv,
 						1 << PIPE_C | 1 << PIPE_B);
 	}
 
-	if (power_well->data == SKL_DISP_PW_1)
+	if (power_well->data == SKL_DISP_PW_1) {
+		intel_prepare_ddi(dev);
 		gen8_irq_power_well_post_enable(dev_priv, 1 << PIPE_A);
+	}
 }
 
 static void hsw_set_power_well(struct drm_i915_private *dev_priv,