drm/i915: Do not clobber the contents of TRANS_DP_CTL when enabling.

Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_reg.h

@@ -2928,6 +2928,7 @@
 #define  TRANS_DP_VSYNC_ACTIVE_LOW	0
 #define  TRANS_DP_HSYNC_ACTIVE_HIGH	(1<<3)
 #define  TRANS_DP_HSYNC_ACTIVE_LOW	0
+#define  TRANS_DP_SYNC_MASK	(3<<3)
 
 /* SNB eDP training params */
 /* SNB A-stepping */

drivers/gpu/drm/i915/intel_display.c

@@ -2097,9 +2097,10 @@ static void ironlake_crtc_dpms(struct drm_crtc *crtc, int mode)
 				int reg;
 
 				reg = I915_READ(trans_dp_ctl);
-				reg &= ~TRANS_DP_PORT_SEL_MASK;
-				reg = TRANS_DP_OUTPUT_ENABLE |
-				      TRANS_DP_ENH_FRAMING;
+				reg &= ~(TRANS_DP_PORT_SEL_MASK |
+					 TRANS_DP_SYNC_MASK);
+				reg |= (TRANS_DP_OUTPUT_ENABLE |
+					TRANS_DP_ENH_FRAMING);
 
 				if (crtc->mode.flags & DRM_MODE_FLAG_PHSYNC)
 				      reg |= TRANS_DP_HSYNC_ACTIVE_HIGH;