[Libva] keep Nalu start code in VASliceDataBufferType data

[Randy Li]

Hi all:
   When I just doing the driver for us chip, we would request the Nalu 
header present in the data to be process. But I found the data be 
Rendered to with type VASliceDataBufferType is removed the Nalu start 
code. Is there any way to make the client send the data without remove 
the start code ? Thank you.

P.S Thank you for the Intel guys help, I decided not to use the DRM 
framework to implement the interface in kernel after I talked to the 
kernel upstream. But the request API would be used.
-- 
Randy Li
The third produce department