drm/i915: Handle GPU hangs during fault gracefully.

Instead of killing the process, just return no page found and reschedule
the process giving the GPU some time to (hopefully) recover.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gem.c

@@ -1340,11 +1340,12 @@ int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 	mutex_unlock(&dev->struct_mutex);
 
 	switch (ret) {
+	case -EAGAIN:
+		set_need_resched();
 	case 0:
 	case -ERESTARTSYS:
 		return VM_FAULT_NOPAGE;
 	case -ENOMEM:
-	case -EAGAIN:
 		return VM_FAULT_OOM;
 	default:
 		return VM_FAULT_SIGBUS;