drm/i915: Fix PCH detect with multiple ISA bridges in VM

In some virtualized environments (e.g. XEN), there is irrelevant ISA bridge in
the system. To work reliably, we should scan trhough all the ISA bridge
devices and check for the first match, instead of only checking the first one.

Signed-off-by: Rui Guo <firemeteor@users.sourceforge.net>
[danvet: Fixup conflict with the num_pch_pll removal. And add
subsystem header to the commit message headline.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>