drm/i915: Just use icl+ definition for PLANE_WM blocks field

The unused bits on PLANE_WM & co. are hardwired to zero. So no
need to worry about reading the extra bit on pre-icl.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190205205056.30081-2-ville.syrjala@linux.intel.com


Reviewed-by: José Roberto de Souza <jose.souza@intel.com>