drm/i915: Introduce vlv_invert_wm_value()

Add a small helper to do invert the vlv/chv values. Less fragile
perhaps, and let's us clearly mark all overlarge wateramarks as
disabled (by just making them all USHRT_MAX).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480354637-14209-8-git-send-email-ville.syrjala@linux.intel.com


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