drm/i915: Downgrade Gen9 Plane WM latency error

According to intel_read_wm_latency() it is perfectly legal for one WM
and all subsequent levels to be 0 (and the deeper powersaving states
disabled), so don't shout *ERROR*, over and over again.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180726161527.10516-1-chris@chris-wilson.co.uk