drm/i915: fix Sphinx indentation warning

Fix Sphinx warning about unexpected indent.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230621123156.14907-2-jani.nikula@intel.com