Hi
Whith drm.debug=1 parameter on kernel command line will see this bug.
On different clients or diferent instances of vlc all is ok , but when try to
start
Visualisations from panel vlc entire system is broken and this issue broken
also and
X server.
New threads is also blocked from creation.
When set nouveau.config=SW=0 (software engine is disabled) only Xserver have
access on drm and alls is ok.
My conclusion is;with different $PID have good acces on drm  but when children
task on vlc try to acces on drm alls is break and break also Xserver.
Same problem is on gnome-shell tasks which request access on drm break entire
system and Xserver.