powerpc/powernv/npu: Use size-based ATSD invalidates

Prior to this change only two types of ATSDs were issued to the NPU:
invalidates targeting a single page and invalidates targeting the whole
address space. The crossover point happened at the configurable
atsd_threshold which defaulted to 2M. Invalidates that size or smaller
would issue per-page invalidates for the whole range.

The NPU supports more invalidation sizes however: 64K, 2M, 1G, and all.
These invalidates target addresses aligned to their size. 2M is a common
invalidation size for GPU-enabled applications because that is a GPU
page size, so reducing the number of invalidates by 32x in that case is a
clear improvement.

ATSD latency is high in general so now we always issue a single invalidate
rather than multiple. This will over-invalidate in some cases, but for any
invalidation size over 2M it matches or improves the prior behavior.
There's also an improvement for single-page invalidates since the prior
version issued two invalidates for that case instead of one.

With this change all issued ATSDs now perform a flush, so the flush
parameter has been removed from all the helpers.

To show the benefit here are some performance numbers from a
microbenchmark which creates a 1G allocation then uses mprotect with
PROT_NONE to trigger invalidates in strides across the allocation.

One NPU (1 GPU):

         mprotect rate (GB/s)
Stride   Before      After      Speedup
64K         5.3        5.6           5%
1M         39.3       57.4          46%
2M         49.7       82.6          66%
4M        286.6      285.7           0%

Two NPUs (6 GPUs):

         mprotect rate (GB/s)
Stride   Before      After      Speedup
64K         6.5        7.4          13%
1M         33.4       67.9         103%
2M         38.7       93.1         141%
4M        356.7      354.6          -1%

Anything over 2M is roughly the same as before since both cases issue a
single ATSD.

Signed-off-by: Mark Hairgrove <mhairgrove@nvidia.com>
Reviewed-By: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>