drm/i915/tv: Preserve reserved DAC bits during mode-setting

Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_reg.h

@@ -1481,6 +1481,7 @@
 # define TV_TEST_MODE_MASK		(7 << 0)
 
 #define TV_DAC			0x68004
+# define TV_DAC_SAVE		0x00ffff00
 /**
  * Reports that DAC state change logic has reported change (RO).
  *

drivers/gpu/drm/i915/intel_tv.c

@@ -1196,7 +1196,7 @@ intel_tv_mode_set(struct drm_encoder *encoder, struct drm_display_mode *mode,
 		I915_WRITE(TV_V_LUMA_0 + (i<<2), tv_mode->filter_table[j++]);
 	for (i = 0; i < 43; i++)
 		I915_WRITE(TV_V_CHROMA_0 + (i<<2), tv_mode->filter_table[j++]);
-	I915_WRITE(TV_DAC, 0);
+	I915_WRITE(TV_DAC, I915_READ(TV_DAC) & TV_DAC_SAVE);
 	I915_WRITE(TV_CTL, tv_ctl);
 }