imx-drm: imx-drm-core: use array instead of list for CRTCs

The DRM core indexes vblank by number, so there's little point
maintaining a list, and have to scan the list to find the appropriate
structure.  Instead, use an array of pointers to the CRTCs.

Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>