drm/i915/selftests: Pretty print engine state when requests fail to start

During hangcheck testing, we try to execute requests following the GPU
reset, and in particular want to try and debug when those fail.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171009110301.21705-2-chris@chris-wilson.co.uk


Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>