drm/i915: enable and disable DDI_FUNC_CTL at the right time

And the right time is exactly after/before changing PIPE_CONF. See the
documentation about the mode set sequence.

This code is not inside any encoder-specific callback because
DDI_FUNC_CTL is part of the pipe, so it is used by all encoders.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 5107cee..d1b58d0 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -4394,6 +4394,7 @@
 /* Those bits are ignored by pipe EDP since it can only connect to DDI A */
 #define  PIPE_DDI_PORT_MASK		(7<<28)
 #define  PIPE_DDI_SELECT_PORT(x)	((x)<<28)
+#define  PIPE_DDI_PORT_NONE		(0<<28)
 #define  PIPE_DDI_MODE_SELECT_MASK	(7<<24)
 #define  PIPE_DDI_MODE_SELECT_HDMI	(0<<24)
 #define  PIPE_DDI_MODE_SELECT_DVI	(1<<24)