drm/i915: Kill level 0 wm hack for VLV/CHV

We now compute the watermarks correctly, so just return an error if we
can't support the configuration.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302171508.1666-15-ville.syrjala@linux.intel.com