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

drivers/gpu/drm/i915/gt/intel_lrc.c

@@ -1169,6 +1169,7 @@ static void process_csb(struct intel_engine_cs *engine)
 	u8 head, tail;
 
 	lockdep_assert_held(&engine->active.lock);
+	GEM_BUG_ON(USES_GUC_SUBMISSION(engine->i915));
 
 	/*
 	 * Note that csb_write, csb_status may be either in HWSP or mmio.