nouveau: Fix alignment requirements on src and dst addresses

Linear copy works by adding the offset to the buffer address,
which may end up not being 16-byte aligned.

Some tests I've written for prime_pcopy show that the engine
allows this correctly, so the restriction on lowest 4 bits of
address can be lifted safely.

The comments added were by envyas, I think because I used
a newer version.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Cc: stable@vger.kernel.org