Replace the 0 and 1 with PLANE_A and PLANE_B in the pre-g4x wm code.

v2: s/old_plane_id/i9xx_plane_id/ (Daniel)
v3: s/plane/i9xx_plane/ etc. (James)

Cc: James Ausmus <james.ausmus@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171117191917.11506-5-ville.syrjala@linux.intel.comSigned-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>