drm/i915: fix guest virtual PCH detection on non-PCH systems

Virtualized non-PCH systems such as Broxton or Geminilake should use
PCH_NONE to indicate no PCH rather than PCH_NOP. The latter is a
specific case to indicate a PCH system without south display.

Reported-by: Colin Xu <Colin.Xu@intel.com>
Cc: Colin Xu <Colin.Xu@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Colin Xu <Colin.Xu@intel.com>
Reviewed-by: Colin Xu <Colin.Xu@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180608123330.31003-2-jani.nikula@intel.com