imx-drm: fix hdmi hotplug detection initial state

The initial state at boot is assumed to be disconnected, and we hope
to receive an interrupt to update the status.  Let's be more explicit
about the current state - reading the PHY status register tells us
the current level of the hotplug signal, which we can report back in
the _detect() method.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>