drm/i915: fix the transition minimums for gen9+ watermarks

The transition minimum is 14 blocks for gens 9 and 10, and 4 blocks
for gen 11. This minimum value is supposed to be added to the
configurable trans_amount. This matches both BSpec and additional
information provided by our HW engineers.
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004231600.14101-3-paulo.r.zanoni@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -4875,8 +4875,8 @@ static void skl_compute_transition_wm(struct intel_crtc_state *cstate,
 	if (!dev_priv->ipc_enabled)
 		goto exit;
 
-	trans_min = 0;
-	if (INTEL_GEN(dev_priv) >= 10)
+	trans_min = 14;
+	if (INTEL_GEN(dev_priv) >= 11)
 		trans_min = 4;
 
 	trans_offset_b = trans_min + trans_amount;