nouveau: compat fix for set_page_locked().

The set_page_locked() function has changed its name again.
2.6.28 offers __set_page_locked() instead, which uses non-atomic
__set_bit() to do the work. In this case, offer our own
set_page_locked() using the atomic set_bit(), because I do not know
if atomic access is really necessary. Atomic behaviour is the one
previously expected.

Signed-off-by: Pekka Paalanen <pq@iki.fi>