drm/i915: Setting pch_id for HSW/BDW in virtual environment

drm/i915: Setting pch_id for HSW/BDW in virtual environment

In a IGD passthrough environment, the real ISA bridge may doesn't exist.
then pch_id couldn't be correctly gotten from ISA bridge, but pch_id is
used to identify LPT_H and LPT_LP. Currently i915 treat all LPT pch as
LPT_H,then errors occur when i915 runs on LPT_LP machines with igd
passthrough.

This patch set pch_id for HSW/BDW according to IGD type and isn't fully
correct. But it solves such issue on HSW/BDW ult/ulx machines.
QA CI system is blocked by this issue for a long time, it's better that
we could merge it to unblock QA CI system.

We know the root cause is in device model of virtual passthrough, and
will resolve it in the future with several parts cooperation in kernel,
qemu and xen.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99938

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1497496305-5364-1-git-send-email-xiong.y.zhang@intel.com