dyndbg: add __pr_debug_cls for testing

dyndbg: add __pr_debug_cls for testing

For selftest purposes, add __pr_debug_cls(class, fmt, ...)

I didn't think we'd need to define this, since DRM effectively has it
already in drm_dbg, drm_devdbg.  But test_dynamic_debug needs it in
order to demonstrate all the moving parts.

Note the __ prefix; its not intended for general use, at least until a
need emerges.  ISTM the drm.debug model (macro wrappers inserting enum
const 1st arg) is the baseline approach.

That said, nouveau might want it for easy use in its debug macros. TBD.

NB: it does require a builtin-constant class, __pr_debug_cls(i++, ...)
is disallowed by compiler.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Link: https://lore.kernel.org/r/20220904214134.408619-14-jim.cromie@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>