drm/amdgpu/ttm: Use more optimal copy packet sizes for copy and fill

Currently when amdgpu copies or fills a buffer,
it uses the maximum byte count supported by the
copy engine (SDMA). This is problematic when the
maximum byte count is not aligned to 256 bytes
because it then can't use all memory channels
optimally and can cause the SDMA to operate in its
slower byte mode (as opposed to the faster dword mode).

For example, when copying a 10 MiB buffer on SDMA v2.4,
we get 5 packets copying 2097151 bytes and 1 packet copying
the remaining 5 bytes. All 6 packets are misaligned and operate
in byte mode.
For this example, the optimal solution would be to have
5 packets each copying 2096896 bytes and 1 last packet to
copy the remaining 1280 bytes, in which case all 6 packets
are aligned to 256 bytes and operate in dword mode.

Let's use the following scheme from now on:

When byte count is dword-aligned and fits a single packet,
just emit a single packet.

Otherwise, align the copy packet size down to 256 bytes
for optimal use of memory channels and to ensure the HW
can use the dword mode.

This assumes that the starting addresses of BOs are always
dword aligned, which should be the case for every copy
operation in the kernel, because the kernel always copies
pages.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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