nouveau: Fix alignment requirements on src and dst addresses

commit ce806a30470bcd846d148bf39d46de3ad7748228 upstream.

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>
[bwh: Backported to 3.2: no # prefixes in nva3_copy.fuc]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>