drm/i915/gtt: Markup i915_ppgtt height

This will be useful to consolidate recursive code.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712094327.24437-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -1598,6 +1598,7 @@ static struct i915_ppgtt *gen8_ppgtt_create(struct drm_i915_private *i915)
 		return ERR_PTR(-ENOMEM);
 
 	ppgtt_init(ppgtt, &i915->gt);
+	ppgtt->vm.top = i915_vm_is_4lvl(&ppgtt->vm) ? 3 : 2;
 
 	/*
 	 * From bdw, there is hw support for read-only pages in the PPGTT.
@@ -2084,6 +2085,7 @@ static struct i915_ppgtt *gen6_ppgtt_create(struct drm_i915_private *i915)
 		return ERR_PTR(-ENOMEM);
 
 	ppgtt_init(&ppgtt->base, &i915->gt);
+	ppgtt->base.vm.top = 1;
 
 	ppgtt->base.vm.allocate_va_range = gen6_alloc_va_range;
 	ppgtt->base.vm.clear_range = gen6_ppgtt_clear_range;

drivers/gpu/drm/i915/i915_gem_gtt.h

@@ -323,6 +323,7 @@ struct i915_address_space {
 	struct i915_page_dma scratch_pt;
 	struct i915_page_dma scratch_pd;
 	struct i915_page_dma scratch_pdp; /* GEN8+ & 48b PPGTT */
+	unsigned int top;
 
 	/**
 	 * List of vma currently bound.