drm/debugfs: Add inline to drm_debugfs_dev_init() to suppres -Wunused-function

When CONFIG_DEBUG_FS is not set -Wunused-function warnings appear,
make the static function inline to suppress that.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202309012114.T8Vlfaf8-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202309012131.FeakBzEj-lkp@intel.com/


Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230901-debugfs-fix-unused-function-warning-v1-1-161dd0902975@riseup.net