drm/i915/gtt: disable 2M pages for pre-gen11

We currently disable THP(Transparent-Huge-Pages) for our shmem objects
due to a performance regression with read BW in some internal
benchmarks. Given that this is our main source of 2M pages, there really
isn't much point in enabling 2M GTT pages, especially as that comes at
the cost of disabling the GTT cache. However from gen11 it looks like we
should hopefully see the HW issue resolved. Given this opt for only
enabling 2M GTT pages from gen11 onwards.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190809193456.3836-2-matthew.auld@intel.com