OMAPDSS: DSS: Cleanup cpu_is_xxxx checks

All the cpu_is checks have been moved to dss_init_features function providing a
much more generic and cleaner interface. The OMAP version and revision specific
initializations in various functions are cleaned and the necessary data are
moved to dss_features structure which is local to dss.c.

Signed-off-by: Chandrabhanu Mahapatra <cmahapatra@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>