drm/bridge: dw-hdmi: add better clock disable control

The video setup path aways sets the clock disable register to a specific
value, which has the effect of disabling the CEC engine.  When we add the
CEC driver, this becomes a problem.

Fix this by only setting/clearing the bits that the video path needs to.

Reviewed-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/E1dcBha-00088l-DE@rmk-PC.armlinux.org.uk