ARM: OMAP: dss-common: fix IGEP's DVI DDC channel

IGEP's DVI connector's DDC pins are connected to OMAP's third i2c bus.

When booting with Device Trees the requested bus number is set to -1
which means that the bus number should be dynamically assigned. So the
third i2c bus has 2 has a bus number.

Since now only DT booting is supported for IGEP boards after commit
06ff74fd ("ARM: OMAP2+: remove legacy support for IGEP boards"), the
i2c bus number has to be changed.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>