drm/i915: add another virtual PCH bridge for passthrough support

Some configs use the P2X type but some use a P3X type PCH, so add that
to the detect_pch function so things work correctly.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Allen Kay <allen.m.kay@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1458160290-16710-1-git-send-email-jbarnes@virtuousgeek.org