gpu: nvgpu: Ensure PDE alignment for GVA spaces

When managing GVA spaces the buddy allocator requires PDE size
alignment. This is to ensure that PTE size in buddies always
remains consistent.

Consider the following hypothetical GVA space: it is 32 elements
long, order 0 block size is 1, and PDE size is 8. This leads to:

  Base: 8
  Size: 24
  Managed space: [8, 32)

The start of the space will be 8 (base must be aligned to a PDE
and we need a hole at the bottom for handling errors). Size is
simply the max, 32, minus what we cut out for the low hole. The
two top level buddies are [8 -> 24), and [24 -> 32).

Now, suppose, instead the base were 4:

  Base: 4
  Size: 28
  Managed space: [4, 32)

The top level buddies would be [4 -> 20), [20 -> 28), and [28 -> 32).
This presents several problems: none of the buddies are PDE aligned
and one top level buddy is smaller than the PDE size. The simplest
issue is how to determine the PTE size of the [28 -> 32) block. We
can just set it as small but that's not ideal. The bigger issue is
the mis-alignment of the larger buddies. [20 -> 28) is halfway in
one PDE and halfway in another. That means the allocator would have
to manage the two sub-buddies [20 -> 24) and [24 -> 28) separately.

Instead of dealing with the above issues in the allocator it is much
more simple to require that any GVA space is PDE aligned since they
are already massive and already, in practice, have this alignment.

Change-Id: I9eacd2db6485291db9f9f1d6c4c03c2a5c22de03
Signed-off-by: Alex Waterman <alexw@nvidia.com>
Reviewed-on: http://git-master/r/1185137
Reviewed-by: Yu-Huan Hsu <yhsu@nvidia.com>

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