drm/i915: Use enum plane_id in VLV/CHV wm code

Let's try not to abuse plane->plane for sprites on VLV/CHV and instead
use plane->id. Since out watermark structures aren't entirely plane type
agnostic (for now) and start indexing sprites from 0  we'll add a small
helper to convert between the two bases.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479830524-7882-7-git-send-email-ville.syrjala@linux.intel.com