drm/i915/gtt: Recursive ppgtt alloc for gen8

Refactor the separate allocation routines into a single recursive
function.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712112725.2892-4-chris@chris-wilson.co.uk