drm/i915: Make vlv/chv watermark debug print less cryptic

The magic numbers 0,1,2 aren't all that interesting for users perhaps.
Since we know what these watermark levels mean for VLV/CHV let's print
their names.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170421181432.15216-6-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>