drm/i915/fence: Mark debug_fence_free() with __maybe_unused

drm/i915/fence: Mark debug_fence_free() with __maybe_unused

When debug_fence_free() is unused
(CONFIG_DRM_I915_SW_FENCE_DEBUG_OBJECTS=n), it prevents kernel builds
with clang, `make W=1` and CONFIG_WERROR=y:

.../i915_sw_fence.c:118:20: error: unused function 'debug_fence_free' [-Werror,-Wunused-function]
  118 | static inline void debug_fence_free(struct i915_sw_fence *fence)
      |                    ^~~~~~~~~~~~~~~~

Fix this by marking debug_fence_free() with __maybe_unused.

See also commit 6863f5643dd7 ("kbuild: allow Clang to find unused static
inline functions for W=1 build").

Fixes: fc1584059d6c ("drm/i915: Integrate i915_sw_fence with debugobjects")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240829155950.1141978-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>