imx-drm: imx-drm-core: use defined constant for number of CRTCs.

We have this definition, there's no reason not to use it.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>