drm/gud: use new debugfs device-centered functions

Replace the use of drm_debugfs_create_files() with the new
drm_debugfs_add_file() function, which center the debugfs files
management on the drm_device instead of drm_minor. Moreover, remove the
debugfs_init hook and add the debugfs files directly on gud_probe(),
before drm_dev_register().

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20221226155029.244355-3-mcanal@igalia.com