drm/mm: Mark drm_mm_interval_tree*() functions with __maybe_unused

drm/mm: Mark drm_mm_interval_tree*() functions with __maybe_unused

The INTERVAL_TREE_DEFINE() uncoditionally provides a bunch of helper
functions which in some cases may be not used. This, in particular,
prevents kernel builds with clang, `make W=1` and CONFIG_WERROR=y:

.../drm/drm_mm.c:152:1: error: unused function 'drm_mm_interval_tree_insert' [-Werror,-Wunused-function]
  152 | INTERVAL_TREE_DEFINE(struct drm_mm_node, rb,
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  153 |                      u64, __subtree_last,
      |                      ~~~~~~~~~~~~~~~~~~~~
  154 |                      START, LAST, static inline, drm_mm_interval_tree)
      |                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix this by marking drm_mm_interval_tree*() functions with __maybe_unused.

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

Fixes: 202b52b7fbf7 ("drm: Track drm_mm nodes with an interval tree")
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/20240829154640.1120050-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>