drm/i915/selftests: Skip aperture remapping selftest where there is no aperture

If there is no mappable aperture, we cannot remap it for access, and the
selftest is void.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210412090526.30547-2-matthew.auld@intel.com