ARM: shmobile: r8a7791: koelsch: Fix pinmux for HDMI

The signals output by the DU to the HDMI transmitter use full 24-bit
RGB. Make sure all pins are properly muxed.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>