drm/i915: only build i915_debugfs.c when CONFIG_DEBUG_FS is enabled

The whole file is wrapped around in #if defined(CONFIG_DEBUG_FS) anyway,
so skip the file at the build level already.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>