libavutil & opencl: remove opencl default device type

CL_DEVICE_TYPE_DEFAULT makes duplicated device entries for each
platform using av_opencl_get_device_list()

Reviewed-by: Stefano Sabatini <stefasab@gmail.com>
Reviewed-by: highgod0401 <highgod0401@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>