drm/i915: have virtual PCH detection return a PCH id

Simplify intel_virt_detect_pch() by making it return a PCH id rather
than returning the PCH type and setting PCH id for some PCHs. Map the
PCH id to PCH type using the shared routine. This gives us sanity check
on the supported combinations also in the virtualized setting.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/197cf635261a1c628371ffaaee90e8647493af4d.1517851783.git.jani.nikula@intel.com