drm/i915/glk: Fix watermark computations for third sprite plane

Geminilake has a third sprite plane (or fourth universal plane) that is
independent from the cursor. Make sure that for_each_plane_id_on_crtc()
is aware of that extra plane so that the watermark code takes it into
account.

Fixes: e9c9882 ("drm/i915/glk: Configure number of sprite planes properly")
Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: <drm-intel-fixes@lists.freedesktop.org>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170223071600.14356-2-ander.conselvan.de.oliveira@intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -293,6 +293,7 @@ enum plane_id {
 	PLANE_PRIMARY,
 	PLANE_SPRITE0,
 	PLANE_SPRITE1,
+	PLANE_SPRITE2,
 	PLANE_CURSOR,
 	I915_MAX_PLANES,
 };