drm/imx: convert imx-drm to use the generic DRM OF helper

Use the generic DRM OF helper to locate the possible CRTCs for the
encoder, thereby shrinking the imx-drm driver some more.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>