drm/i915/selftests: Check that gpu reset is usable from atomic context

GPU reset is now available with GuC enabled, so re-enable our check that
this reset is usable from atomic context.

Signed-off-by: Fernando Pacheco <fernando.pacheco@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190419230015.18121-6-fernando.pacheco@intel.com