drm/rcar-du: Rename platform data fields to match what they describe

The struct rcar_du_encoder_data encoder::field describes the encoder
type, and the rcar_du_encoder_lvds_data and rcar_du_encoder_vga_data
structures describe connector properties. Rename them accordingly.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>