drm/i915: Fix latency==0 handling for level 0 watermark on skl+

If the level 0 latency is 0 we can't do anything. Return an error
rather than success.

While this can't happen due to WaWmMemoryReadLatency, it can
happen if the user clears out the level 0 latency via debugfs.

v2: Clarify how how we can end here with zero level 0 latency (Matt)

Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181114210729.16185-6-ville.syrjala@linux.intel.comReviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/i915/intel_pm.c

@@ -4743,8 +4743,10 @@ static int skl_compute_plane_wm(const struct drm_i915_private *dev_priv,
 	bool apply_memory_bw_wa = skl_needs_memory_bw_wa(state);
 	uint32_t min_disp_buf_needed;
 
-	if (latency == 0 ||
-	    !intel_wm_plane_visible(cstate, intel_pstate)) {
+	if (latency == 0)
+		return level == 0 ? -EINVAL : 0;
+
+	if (!intel_wm_plane_visible(cstate, intel_pstate)) {
 		result->plane_en = false;
 		return 0;
 	}