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

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

When debug_fence_init_onstack() is unused (CONFIG_DRM_I915_SELFTEST=n),
it prevents kernel builds with clang, `make W=1` and CONFIG_WERROR=y:

.../i915_sw_fence.c:97:20: error: unused function 'debug_fence_init_onstack' [-Werror,-Wunused-function]
   97 | static inline void debug_fence_init_onstack(struct i915_sw_fence *fence)
      |                    ^~~~~~~~~~~~~~~~~~~~~~~~

Fix this by marking debug_fence_init_onstack() with __maybe_unused.

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

Fixes: 214707fc2ce0 ("drm/i915/selftests: Wrap a timer into a i915_sw_fence")
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/20240829155950.1141978-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>