codecs: Fix the H265 poc out of order warning.

We always get a warning such as: h265decoder gsth265decoder.c:1432:gst_h265_decoder_do_output_picture:
Outputting out of order 255 -> 0, likely a broken stream in H265 decoder.

The problem is caused because we fail to clear the DPB when we meet IDR and BLA. According to the spec, when we meet the IRAP(except the CRA), we should clean the DPB and set the poc to 0 again. The current way just clean the DPB when NoOutputOfPriorPicsFlag is set to TRUE, which is not correct. NoOutputOfPriorPicsFlag only controls whether we should or not output the pictures before the IRAP frame. It does not change the behaviour of cleaning the DPB.