imx-drm: imx-drm-core: add core hotplug connector support

Add core imx-drm support for hotplug connector support.  We need to
setup the poll helper after we've setup the connectors; the helper
scans the connectors to determine their capabilities.

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>