drm/i915/selftests: Fix plain use of integer 0 as NULL

Quelch a sparse warning:
drivers/gpu/drm/i915/gt/selftest_lrc.c:119:54: warning: Using plain integer as NULL pointer

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190405111430.18495-1-chris@chris-wilson.co.uk