imx-drm: provide common connector mode validation function

imx-drm: provide common connector mode validation function

Provide a common connector mode validation function, which can be used
to limit the available modes according to other components in the
system.

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>