hevc_enc: clear reference lists every frame
hevc_enc: clear reference lists every frame

Because I frames need not to reference to other frames.

Signed-off-by: dongping wu <eada3ae4a96a673269eca90300aaefadacf3db15@intel.com>

01org/libyami,YuJiankang/libyami,01org/libyami,01org/libyami,YuJiankang/libyami,YuJiankang/libyami