drm/i915: Always set the DP transcoder config to 8BPC.

The pipe is always set to 8BPC, but here we were leaving whatever
previous bits were set by the BIOS in place.
Signed-off-by: Eric Anholt <eric@anholt.net>
Tested-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org

drivers/gpu/drm/i915/i915_reg.h

@@ -3033,6 +3033,7 @@
 #define  TRANS_DP_10BPC		(1<<9)
 #define  TRANS_DP_6BPC		(2<<9)
 #define  TRANS_DP_12BPC		(3<<9)
+#define  TRANS_DP_BPC_MASK	(3<<9)
 #define  TRANS_DP_VSYNC_ACTIVE_HIGH	(1<<4)
 #define  TRANS_DP_VSYNC_ACTIVE_LOW	0
 #define  TRANS_DP_HSYNC_ACTIVE_HIGH	(1<<3)

drivers/gpu/drm/i915/intel_display.c

@@ -2120,9 +2120,11 @@ static void ironlake_crtc_enable(struct drm_crtc *crtc)
 		reg = TRANS_DP_CTL(pipe);
 		temp = I915_READ(reg);
 		temp &= ~(TRANS_DP_PORT_SEL_MASK |
-			  TRANS_DP_SYNC_MASK);
+			  TRANS_DP_SYNC_MASK |
+			  TRANS_DP_BPC_MASK);
 		temp |= (TRANS_DP_OUTPUT_ENABLE |
 			 TRANS_DP_ENH_FRAMING);
+		temp |= TRANS_DP_8BPC;
 
 		if (crtc->mode.flags & DRM_MODE_FLAG_PHSYNC)
 			temp |= TRANS_DP_HSYNC_ACTIVE_HIGH;