drm/i915: Probe for PCH chipset type

PCH is the new name for south bridge from Ironlake/Sandybridge,
which contains most of the display outputs except eDP. This one
adds a probe function to detect current PCH type, and method to
detect Cougarpoint PCH.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>