imx-drm: imx-hdmi: add hotplug support to HDMI component

Add hotplug support.  We have to make the interrupt handler threaded so
we can call drm_helper_hpd_irq_event().  Keeping in mind that we will
want to share the interrupt with other HDMI interface drivers (eg, audio
and CEC) put the groundwork in now for that, rather than just using
IRQF_ONESHOT.

Also, we must not call drm_helper_hpd_irq_event() until we have fully
setup the connector; keep the interrupt(s) muted until after that point.

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>