drm/i915: Tidy up GEM_TRACE around execlists

Looking at the coordination of resets with the submission of execlists,
it will be useful to have a GEM_TRACE for when we issue the reset.
Whilst there tidy up the other GEM_TRACE to always include the engine
name, and be careful not to trust any pointers prior to asserts.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171220090626.31643-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_lrc.c

@@ -504,7 +504,7 @@ static void inject_preempt_context(struct intel_engine_cs *engine)
 	ce->ring->tail &= (ce->ring->size - 1);
 	ce->lrc_reg_state[CTX_RING_TAIL+1] = ce->ring->tail;
 
-	GEM_TRACE("\n");
+	GEM_TRACE("%s\n", engine->name);
 	for (n = execlists_num_ports(&engine->execlists); --n; )
 		elsp_write(0, engine->execlists.elsp);
 
@@ -906,7 +906,7 @@ static void execlists_submission_tasklet(unsigned long data)
 			GEM_TRACE("%s out[0]: ctx=%d.%d, seqno=%x\n",
 				  engine->name,
 				  port->context_id, count,
-				  rq->global_seqno);
+				  rq ? rq->global_seqno : 0);
 			GEM_BUG_ON(count == 0);
 			if (--count == 0) {
 				GEM_BUG_ON(status & GEN8_CTX_STATUS_PREEMPTED);
@@ -1555,6 +1555,8 @@ static void reset_common_ring(struct intel_engine_cs *engine,
 	struct intel_context *ce;
 	unsigned long flags;
 
+	GEM_TRACE("%s seqno=%x\n",
+		  engine->name, request ? request->global_seqno : 0);
 	spin_lock_irqsave(&engine->timeline->lock, flags);
 
 	/*