drm/i915/guc: Disable irq for __i915_guc_submit wq_lock

__i915_guc_submit may be, despite my assertion, called from outside of
an irq-safe spinlock so we need to use a full spin_lock_irqsave and not
cheat using a spin_lock. (The initial notify callback from the completed
fence is called before the spinlock is taken to wake up all waiters and
call their callbacks.)

[   48.166581] kernel BUG at drivers/gpu/drm/i915/i915_guc_submission.c:527!
[   48.166617] invalid opcode: 0000 [#1] PREEMPT SMP
[   48.166644] Modules linked in: i915 prime_numbers x86_pkg_temp_thermal intel_powerclamp coretemp crct10dif_pclmul crc32_pclmul ghash_clmulni_intel mei_me mei i2c_i801 netconsole i2c_hid [last unloaded: i915]
[   48.166733] CPU: 2 PID: 5 Comm: kworker/u8:0 Tainted: G     U          4.10.0nightly-170302-guc_scrub+ #19
[   48.166778] Hardware name:                  /NUC6i5SYB, BIOS SYSKLi35.86A.0054.2016.0930.1102 09/30/2016
[   48.166835] Workqueue: i915 __intel_autoenable_gt_powersave [i915]
[   48.166865] task: ff...

drivers/gpu/drm/i915/i915_guc_submission.c

@@ -515,6 +515,7 @@ static void __i915_guc_submit(struct drm_i915_gem_request *rq)
 	unsigned int engine_id = engine->id;
 	struct intel_guc *guc = &rq->i915->guc;
 	struct i915_guc_client *client = guc->execbuf_client;
+	unsigned long flags;
 	int b_ret;
 
 	/* WA to flush out the pending GMADR writes to ring buffer. */
@@ -523,10 +524,7 @@ static void __i915_guc_submit(struct drm_i915_gem_request *rq)
 
 	trace_i915_gem_request_in(rq, 0);
 
-	/* We are always called with irqs disabled */
-	GEM_BUG_ON(!irqs_disabled());
-
-	spin_lock(&client->wq_lock);
+	spin_lock_irqsave(&client->wq_lock, flags);
 
 	guc_wq_item_append(client, rq);
 	b_ret = guc_ring_doorbell(client);
@@ -539,7 +537,7 @@ static void __i915_guc_submit(struct drm_i915_gem_request *rq)
 	guc->submissions[engine_id] += 1;
 	guc->last_seqno[engine_id] = rq->global_seqno;
 
-	spin_unlock(&client->wq_lock);
+	spin_unlock_irqrestore(&client->wq_lock, flags);
 }
 
 static void i915_guc_submit(struct drm_i915_gem_request *rq)