drm: rcar-du: Refactor Feature and Quirk definitions

These flags are represented by bit fields. To make this clear, utilise
the BIT() macro.

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>