drm/i915: move i915_cache_level_str() static in i915_debugfs.c

Move the function next to the only user.

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/dc0901dbe424c21b3e03b875bf5b944b214d1af4.1644507885.git.jani.nikula@intel.com