drm/i915: Remove unused ppgtt->enable()

We never assign or use the ppgtt->enable() callback, so remove it.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215084357.19977-19-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.h

@@ -385,7 +385,6 @@ struct i915_hw_ppgtt {
 
 	gen6_pte_t __iomem *pd_addr;
 
-	int (*enable)(struct i915_hw_ppgtt *ppgtt);
 	int (*switch_mm)(struct i915_hw_ppgtt *ppgtt,
 			 struct drm_i915_gem_request *req);
 	void (*debug_dump)(struct i915_hw_ppgtt *ppgtt, struct seq_file *m);