drm/i915: abstract PCH type detection from PCH id

Make the logic in intel_detect_pch() easier to follow, and make the PCH
id to type mapping reusable. No functional changes.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3bd4ffcd284cdbd4e8dc77ab02d97ded422e0c21.1517851783.git.jani.nikula@intel.com