drm/i915: Also clear the punit's PDATA sideband register

As the previous punit i/o may have failed, the contents of the PDATA are
undefined. Always clear it to 0 prior to sending the command.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170125134808.6152-2-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>

drivers/gpu/drm/i915/intel_sideband.c

@@ -60,8 +60,7 @@ static int vlv_sideband_rw(struct drm_i915_private *dev_priv, u32 devfn,
 	}
 
 	I915_WRITE(VLV_IOSF_ADDR, addr);
-	if (!is_read)
-		I915_WRITE(VLV_IOSF_DATA, *val);
+	I915_WRITE(VLV_IOSF_DATA, is_read ? 0 : *val);
 	I915_WRITE(VLV_IOSF_DOORBELL_REQ, cmd);
 
 	if (intel_wait_for_register(dev_priv,
@@ -74,7 +73,6 @@ static int vlv_sideband_rw(struct drm_i915_private *dev_priv, u32 devfn,
 
 	if (is_read)
 		*val = I915_READ(VLV_IOSF_DATA);
-	I915_WRITE(VLV_IOSF_DATA, 0);
 
 	return 0;
 }