va: Add and use common decode negotiate vmethod.

va: Add and use common decode negotiate vmethod.

This vmethod can be used by decoders with the same VA decoder reopen logic:
same profile, chroma, width and height.

Also a new public method called gst_va_base_dec_set_output_state() with the
common GStreamer code for setting the output state, which is always called by
the negotiate vmethod.

In order to do this refactoring, new variables in vabasedec have to be populated
by the decoders:

* width and height define the resolution set in VA decoder. In the case of H264
  would be de coded_width and codec_height, or max_width and max_height in AV1.
* output_info is the downstream video info used for negotiation in
  gst_va_base_dec_set_output_state().
* input_state, from codec parent class shall be also held by vabasedec

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3480>