drm: rcar-du: Rename DORCR fields to make them 0-based

The DORCR fields were documented in the R-Car H1 datasheet with 1-based
named, and then got renamed to 0-based in Gen2. The 0-based names are
used for Gen3 and Gen4, making H1 an outlier. Rename the field macros to
make them 0-based, in order to increase readability of the code when
comparing it with the documentation.

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