v4l: xilinx-gamma: Correct default values when max width, height dt property absent

v4l: xilinx-gamma: Correct default values when max width, height dt property absent

Revert to 3840x2160 as default value of max width and height to
maintain compatibility instead of 8192x4320.

Signed-off-by: Vishal Sagar <vishal.sagar@xilinx.com>
Reviewed-by: Satish Kumar Nagireddy <satish.nagireddy.nagireddy@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>