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/896Signed-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