drm/i915/dsb: Use non-posted register writes for legacy LUT

The DSB has problems writing the legacy LUT. The two workarounds
I've discoverted are:
- write each entry twice back to back
- use non-posted writes

Let's use non-posted writes as that seems a bit more standard.

TODO: measure which is faster
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230606191504.18099-14-ville.syrjala@linux.intel.comReviewed-by: Uma Shankar <uma.shankar@intel.com>

drivers/gpu/drm/i915/display/intel_color.c

@@ -1329,9 +1329,20 @@ static void ilk_load_lut_8(const struct intel_crtc_state *crtc_state,
 
 	lut = blob->data;
 
+	/*
+	 * DSB fails to correctly load the legacy LUT
+	 * unless we either write each entry twice,
+	 * or use non-posted writes
+	 */
+	if (crtc_state->dsb)
+		intel_dsb_nonpost_start(crtc_state->dsb);
+
 	for (i = 0; i < 256; i++)
 		ilk_lut_write(crtc_state, LGC_PALETTE(pipe, i),
 			      i9xx_lut_8(&lut[i]));
+
+	if (crtc_state->dsb)
+		intel_dsb_nonpost_end(crtc_state->dsb);
 }
 
 static void ilk_load_lut_10(const struct intel_crtc_state *crtc_state,