drm: xilinx: drv: Fix the format descriptor

Fix the Xilinx RGB format to be 24 bit RGB format, and add one more descriptor
for XRGB format with 24 bit depth and 32 bit pixel.

Signed-off-by: Hyun Kwon <hyunk@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>