drm: omapdrm: Move FEAT_LCD_CLK_SRC feature to dss_features structure

The FEAT_LCD_CLK_SRC feature is specific to the DSS, move it from the
omap_dss_features structure to the dss_features structure.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>