drm/radeon/kms: use udelay for short delays

For usec delays use udelay instead of scheduling, this should
allow reclocking to happen faster. This also was the cause
of reported 33s delays at bootup on certain systems.

fixes: freedesktop.org bug 25506

Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>