drm/i915: Assert that we don't submit to execlists whilst a preempt is pending

To complement the check in execlists_elsp_ready(), also assert that we
don't submit the same context while it has a lite restore still pending.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170124110009.28947-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_lrc.c

@@ -375,6 +375,7 @@ static void execlists_submit_ports(struct intel_engine_cs *engine)
 		dev_priv->regs + i915_mmio_reg_offset(RING_ELSP(engine));
 	u64 desc[2];
 
+	GEM_BUG_ON(port[0].count > 1);
 	if (!port[0].count)
 		execlists_context_status_change(port[0].request,
 						INTEL_CONTEXT_SCHEDULE_IN);