drm/nouveau/pm: don't read/write beyond end of stack buffer

NUL-terminate after strncpy.

If the parameter "profile" has length 16 or more, then strncpy
leaves "string" with no NUL terminator, so the following search
for '\n' may read beyond the end of that 16-byte buffer.
If it finds a newline there, then it will also write beyond the
end of that stack buffer.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>