ARM: dts: r8a779[01]: Disable unconnected LVDS encoders

The LVDS0 encoder on Koelsh and Porter, and the LVDS1 encoder on Lager,
are enabled in DT but have no device connected to their output. This
result in spurious messages being printed to the kernel log such as

rcar-du feb00000.display: no connector for encoder /soc/lvds@feb90000, skipping

Fix it by disabling the encoders.

Fixes: 15a1ff30 ("ARM: dts: r8a7790: Convert to new LVDS DT bindings")
Fixes: e5c3f470 ("ARM: dts: r8a7791: Convert to new LVDS DT bindings")
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>