default or selected kernelversion does not have config value CONFIG_DRM_RCAR_HDMI.
Result is shown for kernelversion 6.4.14
R-Car DU HDMI Encoder Support
configname: CONFIG_DRM_RCAR_HDMI
Linux Kernel Configuration
└─>Device Drivers
└─>Graphics support
└─>R-Car DU HDMI Encoder Support
In linux kernel since version 3.19 (release Date: 2015-02-08)
Enable support for external HDMI encoders.