vdpau: remove in-process instance sharing

Originally, both the decoder and the video output would instantiate
VDPAU devices on their own. The process-global list was necessary to
force paired decoder and output to use the same VDPAU instance: they
were matched by server name and screen number.

Now that all components in a given pipeline obtain the VDPAU instance
from a shared decoder device, this trick is no longer necessary.

Incidentally, VDPAU instances would also harmlessly but nevertheless
unnecessarily be shared between independent pipelines within the same
process using the same X11 screen. We can ignore this.