drm/i915: Advertise ppgtt support type in platform definition

Instead of being hidden in sanitize_enable_ppgtt.
It also seems to be the place to do so nowadays.

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>