drm/i915: Convert the file mutex into a spinlock

Daniel Vetter pointed out that in this case is would be clearer and
cleaner to use a spinlock instead of a mutex to protect the per-file
request list manipulation. Make it so.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index f2ff258..710d59e 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -851,8 +851,8 @@
 };
 
 struct drm_i915_file_private {
-	struct mutex mutex;
 	struct {
+		struct spinlock lock;
 		struct list_head request_list;
 	} mm;
 };