drm/i915: Always use 9 bits of the LPC bridge device ID for PCH detection

Make the code less confusiong by always using the top 9 bits of the
LPC bridge device ID to detect the PCH type. We need to add a bit of
new code for WPT, and we need to adjust the KBP ID as well. All the
other pre-CNP IDs are fine as is.

The virtualization cases I think are fine. These P2X and P3X IDs
actually just look like the old PIIX4 and PIIX3 IDs to me. Not sure
why they're not called PIIX3/4 though. The qemu one has a comment
saying the full ID is 0x2918 which is fine with 9 bits.

v2: Keep the CNP ID as 0xa300 (DK)

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621174944.23306-1-ville.syrjala@linux.intel.com