imx-drm: delay publishing sysfs connector entries

Delay publishing sysfs connector entries until all components have
initialised.  This reduces the probability of generating false hotplug
events when we're uncertain whether the driver can fully initialise.
This also pulls that code out of the individual imx-drm connector
drivers.

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>