drm/i915/gen9: Actually verify WM levels in verify_wm_state()

Thanks to Paulo Zanoni for indirectly pointing this out.

Looks like we never actually added any code for checking whether or not
we actually wrote watermark levels properly. Let's fix that.

Changes since v1:
- Use %u instead of %d when printing WM state mismatches

Signed-off-by: Lyude <cpaul@redhat.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1476480722-13015-10-git-send-email-cpaul@redhat.com