drm/i915: Don't initialize plane_to_crtc_mapping[] on SKL+

We don't use the enum i9xx_plane_id namespace on SKL+ anymore, so
do not initialize the related plane_to_crtc_mapping[] table either.

Actually the only remaining user of that table is the pre-g4x
watermark code, but no harm in initializing the table on all
pre-SKL platforms.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180305174122.17273-1-ville.syrjala@linux.intel.com


Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>