drm/i915: preserve the PBC bits of TRANS_CHICKEN2

Bits 30 and 24:0 are PBC, so don't zero them. Some of the other bits
are being zeroed, but I couldn't find a reason for this, so leave them
as they are for now to avoid regressions.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
[danvet: Delete the redudant #define that Imre spotted in his review.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
index 6f67fa1..e34ad96 100644
--- a/drivers/gpu/drm/i915/intel_pm.c
+++ b/drivers/gpu/drm/i915/intel_pm.c
@@ -3604,9 +3604,14 @@
 	 * downward, on (only) LVDS of some HP laptops with IVY.
 	 */
 	for_each_pipe(pipe) {
-		val = TRANS_CHICKEN2_TIMING_OVERRIDE;
+		val = I915_READ(TRANS_CHICKEN2(pipe));
+		val |= TRANS_CHICKEN2_TIMING_OVERRIDE;
+		val &= ~TRANS_CHICKEN2_FDI_POLARITY_REVERSED;
 		if (dev_priv->fdi_rx_polarity_inverted)
 			val |= TRANS_CHICKEN2_FDI_POLARITY_REVERSED;
+		val &= ~TRANS_CHICKEN2_FRAME_START_DELAY_MASK;
+		val &= ~TRANS_CHICKEN2_DISABLE_DEEP_COLOR_COUNTER;
+		val &= ~TRANS_CHICKEN2_DISABLE_DEEP_COLOR_MODESWITCH;
 		I915_WRITE(TRANS_CHICKEN2(pipe), val);
 	}
 	/* WADP0ClockGatingDisable */