drm/i915/selftests: Err out on coherency if initialisation failed

If gt initialisation failed, we are left with no engines to use for
coherency testing. Currently we bug out, which makes the actual error,
so fail more gracefully instead.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/896


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191227103050.2715402-1-chris@chris-wilson.co.uk