drm/mm: annotate drm_mm_node_scanned_block() with __maybe_unused

drm/mm: annotate drm_mm_node_scanned_block() with __maybe_unused

Clang build with CONFIG_DRM_DEBUG_MM=n, CONFIG_WERROR=y, and W=1 leads to:

  CC [M]  drivers/gpu/drm/drm_mm.o
../drivers/gpu/drm/drm_mm.c:614:20: error: function 'drm_mm_node_scanned_block' is not needed and will not be emitted [-Werror,-Wunneeded-internal-declaration]
static inline bool drm_mm_node_scanned_block(const struct drm_mm_node *node)
                   ^

Fix this by annotating drm_mm_node_scanned_block() with __maybe_unused.

Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240903173437.3212437-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>