drm: xilinx: plane: Do not set background color in DRM_MODE_DPMS_ON

Don't reset the background color but use default value that the
user selects when instantiating the IP core. In the long run
this should be a separate plane property.

Signed-off-by: Christian Kohn <christian.kohn@xilinx.com>
Reviewed-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>