drm/i915: extract hw ppgtt setup/cleanup code

At the moment only cosmetics, but being able to initialize/cleanup
arbitrary ppgtt address spaces paves the way to have more than one of
them ... Just in case we ever get around to implementing real
per-process address spaces. Note that in that case another vfunc for
ppgtt would be beneficial though. But that can wait until the code
grows a second place which initializes ppgtts.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 4be7c68..cfae2ce 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -421,6 +421,7 @@
 			       struct sg_table *st,
 			       unsigned int pg_start,
 			       enum i915_cache_level cache_level);
+	void (*cleanup)(struct i915_hw_ppgtt *ppgtt);
 };