drm/nouveau: use mdelay instead of large udelay constants

ARM cannot handle udelay for more than 2 miliseconds, so we
should use mdelay instead for those.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Dave Airlie <airlied@gmail.com>