drm/nouveau: drop use of DRM_UDELAY

The DRM_UDELAY is a simple wrapper for udealy() and to be consistent
call udelay() direct like in may other places.
This avoids the need to pull in drm_os_linux.h when we later
drop drmP.h uses in nouveau.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: nouveau@lists.freedesktop.org
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>