gpu: drm: xylon: Disabled layer color key transparency

Added functionality to have by default disabled layer color key transparency for
all available layers (planes).

Signed-off-by: Davor Joja <davorjoja@logicbricks.com>
Tested-by: Christian Kohn <christian.kohn@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>