drm/omap: hdmi4: Ensure the device is active during bind

The bind function performs hardware access (in hdmi4_cec_init()) and
thus requires the device to be active. Ensure this by surrounding the
bind function by hdmi_runtime_get() and hdmi_runtime_put() calls.

Fixes: 27d62452 ("drm/omap: dss: Acquire next dssdev at probe time")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181110111654.4387-3-laurent.pinchart@ideasonboard.com