drm/i915_drm.h: fix a typo

 'rbiter' -> 'arbiter'

Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230529112956.2083389-1-suijingfeng@loongson.cn