drm/i915: Organize vlv/chv watermarks by plane_id

Store the vlv/chv watermark values in straight up arrays indexed by
enum plane_id. Avoids a lot of useless checks for the plane type when
we don't have to think which structure member we need to access.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480354637-14209-7-git-send-email-ville.syrjala@linux.intel.com


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