Move common VA display creation code to GstVaapiPluginBase, with the
default display type remaining "any". Also add a "display-changed"
hook so that subclasses could perform additional tasks when/if the
VA display changed, due to a new display type request for instance.
All plug-ins are updated to cope with the new internal APIs.
Introduce a new GstVaapiPluginBase object that will contain all common
data structures and perform all common tasks. First step is to have a
single place to hold VA displays.
While we are at it, also make sure to store and subsequently release
the appropriate debug category for the subclasses.
