drm/i915: Store 9 bits of PCI Device ID for platforms with a LP PCH

Although we use 9 bits of Device ID for identifying PCH, only 8 bits are
stored in dev_priv->pch_id. This makes HAS_PCH_CNP_LP() and
HAS_PCH_SPT_LP() incorrect. Fix this by storing all the 9 bits for the
platforms with LP PCH.

v2: Drop PCH_LPT_LP change (Imre)

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Fixes: commit ec7e0bb3 ("drm/i915/cnp: Add PCI ID for Cannonpoint LP PCH")
Reported-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1497641774-29104-1-git-send-email-dhinakaran.pandiyan@intel.com