drm: omapdrm: Move FEAT_DPI_USES_VDDS_DSI feature to dpi code

The FEAT_DPI_USES_VDDS_DSI feature is specific to the DPI, move it from
the omap_dss_features structure to the dpi code.

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