Improve LinuxKPI scatter list support.

The i915kms driver in Linux 4.9 reimplement parts of the scatter list
functions with regards to performance. In other words there is not so
much room for changing structure layouts and functionality if the
i915kms should be built AS-IS. This patch aligns the scatter list
support to what is expected by the i915kms driver. Remove some
comments not needed while at it.

Obtained from: kmacy @
MFC after: 1 week
Sponsored by: Mellanox Technologies