v4l2codecs: Fix debug assertion in register functions

v4l2codecs: Fix debug assertion in register functions

As now, we warn if the decoder have no support src pixel format, but that
warning is called before the type (hence the debug category) is initialized.
Fix this by moving the debug category init out of the type initialization,
into the register funcitons.

This will fix an assertion that occures in the register function and allow
relevant log to be seen by the users.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1588>