drm/i915/execlists: Detect cross-contamination with GuC

The process_csb routine from execlists_submission is incompatible with
the GuC backend. Add a warning to detect if we accidentally end up in
the wrong spot.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190618110736.31155-1-chris@chris-wilson.co.uk