drm/i915: Use VMA directly for checking tiling parameters

v2: Rename functions to suit their more active role

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471254551-25805-14-git-send-email-chris@chris-wilson.co.uk