drm/i915/selftests: Allow huge_gem_object to kick the shrinker

A new fi-cml-dallium CI machine has 8G and apparently plenty free, yet
fails some selftests with ENOMEM. The failures all seem to be from
huge_gem_object which does not try very hard to allocate memory,
skipping reclaim entirely. Let's try a bit harder and direct reclaim
before failing.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210112020013.19464-1-chris@chris-wilson.co.uk