dim: Add Documentation to i915 specific paths

We should not be nagging, we should almost be thanking for adding
Documentation patches for i915 :)

Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>