drm/i915/gtt: Clean up GGTT probing code

Use less pointers with the probing code, making it much less confusing
to read.

Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>