drm/i915: Quieten sparse warnings for missing prototypes.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c
index 315326d..c52e16f 100644
--- a/drivers/gpu/drm/i915/i915_dma.c
+++ b/drivers/gpu/drm/i915/i915_dma.c
@@ -1902,7 +1902,7 @@
  *   - dev_priv->fmax
  *   - dev_priv->gpu_busy
  */
-DEFINE_SPINLOCK(mchdev_lock);
+static DEFINE_SPINLOCK(mchdev_lock);
 
 /**
  * i915_read_mch_val - return value for IPS use