drm/i915/cnp: Add PCI ID for Cannonpoint LP PCH

The first two bytes of PCI ID for CNP_LP PCH are the same as that of
SPT_LP. We should really be looking at the first 9 bits instead of the
first 8 to identify platforms, although this seems to have not caused any
problems on earlier platforms. Introduce a 9 bit extended mask for SPT and
CNP while not touching the code for any of the other platforms.

v2: (Rodrigo) Make platform agnostic and fix commit message.

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1496434004-29812-2-git-send-email-rodrigo.vivi@intel.com