drm/bridge: dw-hdmi: Remove unused hdmi_enable_overflow_interrupts()

The cable_plugin member never receives an assignment, so it is always
false, which causes hdmi_enable_overflow_interrupts() to never
be called as per the logic below:

	if (hdmi->cable_plugin && hdmi->sink_is_hdmi)
		hdmi_enable_overflow_interrupts(hdmi);

This has been the case since the driver was originally introduced
in commit 9aaf880e

 ("imx-drm: Add mx6 hdmi transmitter support").

Remove the cable_plugin element and the hdmi_enable_overflow_interrupts()
function that is never called.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> # On R-Car H3
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1518812170-22687-1-git-send-email-festevam@gmail.com