drm/i915: Do not program aux plane offsets on gen11+

The PLANE_AUX_OFFSET mmio does not exist on ICL, do not program it.  We'll
still calculate the aux offset as it is required for adjusing x-y offsets.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181026193805.11077-2-dhinakaran.pandiyan@intel.com