vaapi_h264: Fix POC on IDR frames

In H.264 section 8.2.1, we have that "The bitstream shall not contain
data that result in Min(TopFieldOrderCnt, BottomFieldOrderCnt) not
equal to 0 for a coded IDR frame".  This fixes the encoder to always
conform to this - previously the POC values formed an unbroken
sequence, not resetting to zero on IDR frames.

Signed-off-by: Mark Thompson <sw@jkqxz.net>
