OMAP: DSS2: Clean up DISPC color mode validation checks

The supported set of color modes varies for different DISPC pipelines(plane)
and omap version. This makes the checks for validation of a color mode more
complicated as new omap versions are added.

A dss_feature function is created which tells if a color_mode is supported
for a plane on the current omap revision.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>