va: Refactor _format_changed() to _config_is_equal().

va: Refactor _format_changed() to _config_is_equal().

Change gst_va_decoder_format_changed() to
gst_va_decoder_config_is_equal(), which is more similar with other
GStreamer API.

The function call is replaced but it has to be negated because the
return value is the opposite.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2417>