drm/i915: Don't check uv_wm in skl_plane_wm_equals()

The hardware never sees the uv_wm values (apart from
uv_wm.min_ddb_alloc affecting the ddb allocation). Thus there
is no point in comparing uv_wm to determine if we need to
reprogram the watermark registers. So let's check only the
rgb/y watermark in skl_plane_wm_equals(). But let's leave
a comment behind so that the next person reading this doesn't
get as confused as I did when I added this check.

If the ddb allocation ends up changing due to uv_wm
skl_ddb_add_affected_planes() takes care of adding the plane
to the state.

TODO: we should perhaps just eliminate uv_wm from the state
and simply track the min_ddb_alloc for uv instead.

Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200228203552.30273-1-ville.syrjala@linux.intel.comReviewed-by: José Roberto de Souza <jose.souza@intel.com>

drivers/gpu/drm/i915/intel_pm.c

@@ -5404,8 +5404,12 @@ static bool skl_plane_wm_equals(struct drm_i915_private *dev_priv,
 	int level, max_level = ilk_wm_max_level(dev_priv);
 
 	for (level = 0; level <= max_level; level++) {
-		if (!skl_wm_level_equals(&wm1->wm[level], &wm2->wm[level]) ||
-		    !skl_wm_level_equals(&wm1->uv_wm[level], &wm2->uv_wm[level]))
+		/*
+		 * We don't check uv_wm as the hardware doesn't actually
+		 * use it. It only gets used for calculating the required
+		 * ddb allocation.
+		 */
+		if (!skl_wm_level_equals(&wm1->wm[level], &wm2->wm[level]))
 			return false;
 	}