drm: i915: fix build when DEBUG_FS is disabled

This clearly had never gotten tested, probably because you need a fairly
minimal configuration in order to disable DEBUG_FS (several other
options select it).

The dummy inline functions that were used for the no-DEBUG_FS case were
missing the argument names in the declarations.

Fixes: 1dac891c ("drm/i915: Register debugfs interface last")
Reported-and-tested-by: Jörg Otte <jrg.otte@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>