drivers: video: sw_generator: Disable the driver if a real camera presents

Do not enable the video sw generator if a real camera is present. This
helps to save some spaces and to avoid unrelated code going
into the built image.

Signed-off-by: Trung Hieu Le <trunghieu.le@nxp.com>
Signed-off-by: Phi Bang Nguyen <phibang.nguyen@nxp.com>