drm/i915: Silence a sparse warning

drivers/gpu/drm/i915/i915_debugfs.c:2136:3: warning: symbol
'i915_debugfs_files' was not declared. Should it be static?

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>