drm/i915: Restrict the aliasing-ppgtt to the size of the ggtt

The aliasing-ppgtt is not allowed to be smaller than the ggtt, nor
should we advertise it as being any bigger, or else we may get sued for
false advertisement.

Testcase: igt/gem_exec_big
Fixes: 0b718ba1 ("drm/i915/gtt: Downgrade Cherryview back to aliasing-ppgtt")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190902040303.14195-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -2597,6 +2597,8 @@ static int init_aliasing_ppgtt(struct i915_ggtt *ggtt)
 	GEM_BUG_ON(ggtt->vm.vma_ops.unbind_vma != ggtt_unbind_vma);
 	ggtt->vm.vma_ops.unbind_vma = aliasing_gtt_unbind_vma;
 
+	ppgtt->vm.total = ggtt->vm.total;
+
 	return 0;
 
 err_ppgtt: