drm/i915/selftests: Make unbannable contexts for reset handling

drm/i915/selftests: Make unbannable contexts for reset handling

igt_ctx_sseu was caught using bannable contexts, and in the course of
resetting rapidly to run its test, was banned. Don't let ourselves ban
the test!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190218145051.18981-1-chris@chris-wilson.co.uk