drm/i915/selftests: Skip mixed page exhaustion if only small pages available

If we only have 4k pages, we can't mix together different combinations
of hugepages to see if the world burns. However, as the loops did
nothing, we never set err to 0 and reported ENODEV aborting the test.
Teach the test to skip instead.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171107110559.6098-1-chris@chris-wilson.co.uk


Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>