drm/i915: Set the transcoder port to none when disabling DP.

The specs say to do so.
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_reg.h

@@ -3245,6 +3245,7 @@
 #define  TRANS_DP_PORT_SEL_B	(0<<29)
 #define  TRANS_DP_PORT_SEL_C	(1<<29)
 #define  TRANS_DP_PORT_SEL_D	(2<<29)
+#define  TRANS_DP_PORT_SEL_NONE	(3<<29)
 #define  TRANS_DP_PORT_SEL_MASK	(3<<29)
 #define  TRANS_DP_AUDIO_ONLY	(1<<26)
 #define  TRANS_DP_ENH_FRAMING	(1<<18)

drivers/gpu/drm/i915/intel_display.c

@@ -2818,6 +2818,7 @@ static void ironlake_crtc_disable(struct drm_crtc *crtc)
 		reg = TRANS_DP_CTL(pipe);
 		temp = I915_READ(reg);
 		temp &= ~(TRANS_DP_OUTPUT_ENABLE | TRANS_DP_PORT_SEL_MASK);
+		temp |= TRANS_DP_PORT_SEL_NONE;
 		I915_WRITE(reg, temp);
 
 		/* disable DPLL_SEL */