drm/i915/cnp: Don't touch other PCH clock gating bits.

Don't touch other bits. My bad.

I haven't seen any case where those other bits appeard to be
set before we touch it, but it is safe to avoid touching
other bits we weren't told to touch.

Fixes: 0a46ddd5 ("drm/i915/cnp: Wa 1181: Fix Backlight issue")
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170908234534.17986-1-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -8369,7 +8369,8 @@ static void cnp_init_clock_gating(struct drm_i915_private *dev_priv)
 		return;
 
 	/* Wa #1181 */
-	I915_WRITE(SOUTH_DSPCLK_GATE_D, CNP_PWM_CGE_GATING_DISABLE);
+	I915_WRITE(SOUTH_DSPCLK_GATE_D, I915_READ(SOUTH_DSPCLK_GATE_D) |
+		   CNP_PWM_CGE_GATING_DISABLE);
 }
 
 static void cnl_init_clock_gating(struct drm_i915_private *dev_priv)