drm/irq: BUG_ON() -> WARN_ON()

Let's make things a bit easier to debug when things go bad (potentially
under console_lock).

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>