test/avcenc: Remove the dynamic shrink of coded buffer

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
test/avcenc: Fix the incorrect parameter setting for SPS under CBR mode

Currently the parameter setting is incorrect for CBR, which causes that
it will complain the underflow/overflow during encoding.

V1->V2: Follow Haihao's comment to update some incorrect usage about the
frame_bit_rate.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>