media: imx: imx7-media-csi: Init default format with __imx7_csi_video_try_fmt()

Use the __imx7_csi_video_try_fmt() helper function to initialize the
default format at probe time. This improves consistency by using the
same code path for both default initialization and validation at
runtime, and allows dropping the now unused imx7_csi_find_pixel_format()
function.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>