drm: rcar-du: Replace EXT_CTRL_REGS feature flag with generation check

The RCAR_DU_FEATURE_EXT_CTRL_REGS feature flag is missing for H1 only,
which is a first generation device, not a second generation device as
reported in the device information table. Fix the H1 generation and use
generation checks to replace the feature flag.

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