codecs: Fix the H265 poc out of order warning.

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: \
<vah265dec0> Outputting out of order 255 -> 0, likely a broken stream
in H265 decoder.

The problem is caused because we fail to reset the last_output_poc when
we get IDR and BLA. The incoming IDR and BLA frame already bump all the
frames in the DPB, but we forget to reset the last_output_poc, which
make the POC out of order and generate the warning all the time.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2294>