drm/i915: Free write_buf that we allocated with kzalloc.

We use kzalloc to allocate the write_buf that we use for
i2c transfer on hdcp write. But it seems that we are forgetting
to free the memory that is not needed after i2c transfer is
completed.

Reported-by: Brian J Wood <brian.j.wood@intel.com>
Fixes: 2320175feb74 ("drm/i915: Implement HDCP for HDMI")
Cc: Ramalingam C <ramalingam.c@intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: <stable@vger.kernel.org> # v4.17+
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180823205136.31310-1-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/intel_hdmi.c

@@ -943,8 +943,12 @@ static int intel_hdmi_hdcp_write(struct intel_digital_port *intel_dig_port,
 
 	ret = i2c_transfer(adapter, &msg, 1);
 	if (ret == 1)
-		return 0;
-	return ret >= 0 ? -EIO : ret;
+		ret = 0;
+	else if (ret >= 0)
+		ret = -EIO;
+
+	kfree(write_buf);
+	return ret;
 }
 
 static