drm/radeon: replace udelay with mdelay for long timeouts

Some architectures require that delays longer than a few
miliseconds are called through mdelay. This was triggered
on ARM randconfig builds.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>