drm/i915: Turn off pipe CSC when it's not needed

As with pipe gamma we can avoid the potential precision loss from
the pipe csc unit when there is no need to use it. And again
we need the same logic for updating the planes.

v2: Rebase
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190207202146.26423-6-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/intel_color.c

@@ -663,7 +663,8 @@ intel_color_add_affected_planes(struct intel_crtc_state *new_crtc_state)
 		intel_atomic_get_old_crtc_state(state, crtc);
 	struct intel_plane *plane;
 
-	if (new_crtc_state->gamma_enable == old_crtc_state->gamma_enable)
+	if (new_crtc_state->gamma_enable == old_crtc_state->gamma_enable &&
+	    new_crtc_state->csc_enable == old_crtc_state->csc_enable)
 		return 0;
 
 	for_each_intel_plane_on_crtc(&dev_priv->drm, crtc, plane) {
@@ -704,6 +705,7 @@ int intel_color_check(struct intel_crtc_state *crtc_state)
 	struct drm_i915_private *dev_priv = to_i915(crtc_state->base.crtc->dev);
 	const struct drm_property_blob *gamma_lut = crtc_state->base.gamma_lut;
 	const struct drm_property_blob *degamma_lut = crtc_state->base.degamma_lut;
+	bool limited_color_range = false;
 	int gamma_length, degamma_length;
 	u32 gamma_tests, degamma_tests;
 	int ret;
@@ -717,7 +719,11 @@ int intel_color_check(struct intel_crtc_state *crtc_state)
 
 	if (INTEL_GEN(dev_priv) >= 9 ||
 	    IS_BROADWELL(dev_priv) || IS_HASWELL(dev_priv))
-		crtc_state->csc_enable = true;
+		limited_color_range = crtc_state->limited_color_range;
+
+	crtc_state->csc_enable =
+		crtc_state->output_format != INTEL_OUTPUT_FORMAT_RGB ||
+		crtc_state->base.ctm || limited_color_range;
 
 	ret = intel_color_add_affected_planes(crtc_state);
 	if (ret)