display: refine the meaning of display type.

display: refine the meaning of display type.

Make gst_vaapi_display_get_display_type() return the actual VA display
type. Conversely, add a gst_vaapi_display_get_class_type() function to
return the type of the GstVaapiDisplay instance. The former is used to
identify the display server onto which the application is running, and
the latter to identify the original object class.