drm: xilinx: encoder: Use to_i2c_driver()

struct i2c_client has been changed. Use to_i2c_driver() to get
struct i2c_driver which is used to get struct drm_i2c_encoder_driver.

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