drm/i915/selftests: Report ENOMEM clearly for an allocation failure

drm/i915/selftests: Report ENOMEM clearly for an allocation failure

If we can not run the drunk_hole test because we couldn't allocate the
memory for the permutation array (even after we tried trimming the
size), report a clear ENOMEM. Similary, if we are asked to operate on a
hole too small for ourselves, make it skip quietly.

v2: Avoid malloc(0) since that returns ZERO_SIZE_PTR not NULL.
v3: Fixup similar construction for lowlevel_hole
v4: Use u64 >> 1 to avoid 64b div.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171117101732.4335-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171117162945.16390-1-chris@chris-wilson.co.uk