OMAP: DSS2: Use dss_features to handle DISPC bits removed on OMAP4

DISPC_CONFIG bits LCDENABLEPOL, LCDENABLESIGNAL, PCKFREEENABLE and FUNCGATED
have been removed from OMAP4, use dss_features to handle them correctly.

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