imx-drm: ipu-dc: Use usleep_range instead of msleep

Since msleep(2) can sleep up to 20ms anyway, make this explicit by using
usleep_range(2000, 20000).

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>