The driver doesn't set an initial video format until s_input is called:

  $ v4l2-ctl -d /dev/v4l-touch0 --get-input
  Video input : 0 (Normalized 16-Bit Image: ok)
  $ v4l2-ctl -d /dev/v4l-touch0 --get-fmt-video
  	Width/Height      : 0/0
  	Pixel Format      : ''
  	[...]
  $ v4l2-ctl -d /dev/v4l-touch0 --set-input 0
  Video input set to 0 (Normalized 16-Bit Image: Touch, ok)
  $ v4l2-ctl -d /dev/v4l-touch0 --get-fmt-video
  	Width/Height      : 71/40
  	Pixel Format      : 'TD16'
  	[...]

To fix this, initialize the video format to input 0 during probe.
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>