drm/i915: use vma->size for appgtt allocate_va_range

drm/i915: use vma->size for appgtt allocate_va_range

For the aliasing ppgtt we clear the va range up to vma->size, but seem
to allocate up to vma->node.size, which is a little inconsistent given
that vma->node.size >= vma->size. Not that is really matters all that
much since we preallocate anyway, but for consistency just use
vma->size.

Fixes: ff685975d97f ("drm/i915: Move allocate_va_range to GTT")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170516085514.5853-1-matthew.auld@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>