i915/gem_pread/pwrite: Skip gtt test on self for missing mappable gtt

Some tests are designed to use GGTT API which should be skipped
when HW doesn't support mappable aperture.

Gem pread / pwrite "self" test is one of them.

Signed-off-by: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>