drm/i915: Pass modifier instead of tiling_mode to gen4_compute_page_offset()

In preparation for handling more than X tiling, pass the fb modifier to
gen4_compute_page_offset() instead of the obj->tiling_mode.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1452625717-9713-2-git-send-email-ville.syrjala@linux.intel.com