Currently, assigning new buffer for pkt when multiple buffers were returned
from vaMapBuffer will overwrite the previous encoded pkt data and lead
to encode issues.

Iterate through the buf_list first to find out the total buffer size
needed for the pkt, allocate the whole pkt to avoid repeated reallocation
and memcpy, then copy data from each buf to pkt.
Signed-off-by: Linjie Fu <linjie.fu@intel.com>