Hi,
The driver implements the FBIOPUTCMAP ioctl. But since the framebuffer is in truecolor format (which means you can specify the color for each pixel individually) setting a palette makes no sense and the driver will refuse to load any palette that is larger than 16 entries. 16 is the size of the pseudocolor palette that is used by the framebuffer console.
drivers/gpu/drm/xilinx/ is the driver for the Xilinx video HDL cores, while drivers/gpu/drm/adi_axi_hdmi/ is the driver for the ADI provided video transmitter HDL core, those two are completely independent and not related. If you are using the ADI reference design the later is the correct driver.
- Lars