drm/i915: Use enum plane_id in SKL plane code

Replace the intel_plane->plane and hardcoded 0 usage in the SKL plane
code with intel_plane->id.

This should make the SKL "primary" and "sprite" code virtually
identical, so the next logical step would likely be dropping one
of the copies.

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/1479830524-7882-5-git-send-email-ville.syrjala@linux.intel.com