va: Fix a latent race condition in vabasedec.

va: Fix a latent race condition in vabasedec.

The vabasedec's display and decoder are created/destroyed between
the gst_va_base_dec_open/close pair. All the data and event handling
functions are between this pair and so the accessing to these pointers
are safe. But the query function can be called anytime. So we need to:
1. Make these pointers operation in open/close and query atomic.
2. Hold an extra ref during query function to avoid it destroyed.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1957>