decoder: optimize slice data buffers initialization.

decoder: optimize slice data buffers initialization.

VA drivers may have a faster means to transfer user buffers to GPU
buffers than using memcpy(). In particular, on Intel Gen graphics, we
can use pwrite(). This provides for faster upload of bitstream and can
help higher bitrates.

vaapi_create_buffer() helper function was also updated to allow for
un-mapped buffers and pre-initialized data for buffers.