Improve check for display cache infrastructure. In particular, for X11 and
GLX backends, we need to make sure that we can create a GstVaapiDisplayX11
from another GstVaapiDisplayGLX, i.e. underlying X11 and VA displays can be
shared. Besides, allocating a GstVaapiDisplayGLX while a GstVaapiDisplayX11
already exists will have to generate different VA displays.