Calculate the required space of batch buffer to avoid buffer overflow in encoding

Calculate the required space of batch buffer to avoid buffer overflow in encoding

The required size is based on the number of macroblocks and slice parameter.
Then it can avoid that too large buffer is allocated or possible overflow.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
(cherry picked from commit 8acdfd023e50af37a5642e2517683c34accd78b0)