drm/i915: print warning if vmi915_gem_fault error is not handled

Falling into default case in vmi915_gem_fault is a bug. Be more
verbose about it.

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -1409,6 +1409,7 @@ out:
 	case -ENOMEM:
 		return VM_FAULT_OOM;
 	default:
+		WARN_ON_ONCE(ret);
 		return VM_FAULT_SIGBUS;
 	}
 }