drm: gpu: xilinx: encoder: Use struct device in the encoder structure

Use the generic struct device instead of using the device type specific
structures such as struct i2c_client or struct platform_device.

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