drm/i915/gen8: Un-hardcode number of page directories

Signed-off-by: NBen Widawsky <ben@bwidawsk.net>
Signed-off-by: NMichel Thierry <michel.thierry@intel.com>
Reviewed-by: NMika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.h

@@ -282,7 +282,7 @@ struct i915_hw_ppgtt {
 	};
 	union {
 		dma_addr_t *pt_dma_addr;
-		dma_addr_t *gen8_pt_dma_addr[4];
+		dma_addr_t *gen8_pt_dma_addr[GEN8_LEGACY_PDPES];
 	};
 
 	struct drm_i915_file_private *file_priv;