lavc/vaapi_encode_h264: fix poc incorrect issue after meeting idr frame.

when meeting IDR frame, vaapi_encode_h264 poc number don't reset, now fix
this issue based on h264 spec. Some decoder don't care this case, but this
fix will enhance the encoder action. Before this fix, poc number is
negative in some case.

Reviewed-by: Jun Zhao <jun.zhao@intel.com>
Signed-off-by: Wang, Yi A <yi.a.wang@intel.com>
Signed-off-by: Mark Thompson <sw@jkqxz.net>