vaapidecode: call purge at flush()

vaapidecode: call purge at flush()

Calling flush() vmethod means "to flush all remaining data from the decoder
without pushing it downstream".

Nonetheless flush() is calling gst_vaapidecode_internal_flush(), which calls
gst_video_decoder_have_frame() if there is still something in the input
adapter, which may push buffers to downstream by calling handle_frame().

This patch changes this behavior by calling gst_vaapidecode_purge() rather
than gst_vaapidecode_internal_flush(), which does what we want: flushes the VA
decoder and releases all the rest of decoded frames.

https://bugzilla.gnome.org/show_bug.cgi?id=768652