drm: Remove unused function rcar_cmm_write

drm: Remove unused function rcar_cmm_write

The function are defined in the rcar_cmm.c file, but not called
elsewhere, so delete the unused function.

drivers/gpu/drm/renesas/rcar-du/rcar_cmm.c:35:19: warning: unused function 'rcar_cmm_read'.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=9364
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/20240619075436.86407-1-jiapeng.chong@linux.alibaba.com
Signed-off-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>