drm/i915: Pass the correct plane index to _intel_compute_tile_offset()

intel_fill_fb_info() should pass the correct plane index to
_intel_compute_tile_offset() once we start to care about the AUX
surface.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170307194210.13400-5-ville.syrjala@linux.intel.com