drm/i915/gtt: Compute the radix for gen8 page table levels

The radix levels of each page directory are easily determined so replace
the numerous hardcoded constants with precomputed derived constants.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712094327.24437-4-chris@chris-wilson.co.uk