drm/i915/selftests: Query the vm under test for hugepage support

Since we have the ppgtt we want to test, we can ask it directly if it is
suitable for the hugepage test we intend to undertake.

v2: Not everyone has full-ppgtt

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190117230512.4789-1-chris@chris-wilson.co.uk