I think maybe Opencv do not support I420 data format input. Please try this:
v4l2src ! video/x-raw, format=(string)UYVY, width=(int)1920, height=(int)1080 ! nvvidconv ! video/x-raw(memory:NVMM), format=(string)I420 ! nvvidconv ! video/x-raw, format=(string)BGRx ! videoconvert ! video/x-raw, format=(string)BGR ! appsink