drm/i915: don't write PLANE_BUF_CFG twice every time

We were writing to PLANE_BUF_CFG(pipe, plane_id) twice for every
platform, and we were even using different values on the gen10- planar
case. The first write is useless since it just gets replaced with the
next one, so kill it.

There's a lot to improve in the DDB code, but let's start by avoiding
the double write.
Reviewed-by: NMatt Roper <matthew.d.roper@intel.com>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004231600.14101-6-paulo.r.zanoni@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -5037,8 +5037,6 @@ static void skl_write_plane_wm(struct intel_crtc *intel_crtc,
 	skl_write_wm_level(dev_priv, PLANE_WM_TRANS(pipe, plane_id),
 			   &wm->trans_wm);
 
-	skl_ddb_entry_write(dev_priv, PLANE_BUF_CFG(pipe, plane_id),
-			    &ddb->plane[pipe][plane_id]);
 	/* FIXME: add proper NV12 support for ICL. */
 	if (INTEL_GEN(dev_priv) >= 11)
 		return skl_ddb_entry_write(dev_priv,