drm/i915: fix documentation build warnings

Just a straightforward bag of fixes for a clean htmldocs build.

Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190605095657.23601-2-jani.nikula@intel.com