v4l2videodec: Implement stable element names

v4l2videodec: Implement stable element names

Before that, each m2m node would be wrapped as a single, multi-format
decoder element. As a unique name was needed, we where using the device
name, which changes between re-boots. This led to unpredictable element
names. In this patch, we generate an element per codec, using
v4l2<codec>dec name. If there is multiple decoder for the same format,
the following elements will be named v4l2<node><codec>dec.

https://bugzilla.gnome.org/show_bug.cgi?id=784908