DSS2: pass the default FB color format through board info

Add a field to the FB memory region platform data, so that board
init code can pass a default color format to the driver. Set this
format as an initial setting for the given FB.

This is needed for an upcoming patch that adds detection of the
color format set by the bootloader.

Signed-off-by: Imre Deak <imre.deak@nokia.com>