drm/i915: clean up virtual PCH special case handling

Use intel_pch_type() also for mapping the no PCH case (PCH id 0) to
PCH_NONE to simplify code.

Also make sure that intel_pch_type() knows all the PCH ids returned by
intel_virt_detect_pch(). Loudly fail if this isn't the case; this
shouldn't happen anyway.

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/20180608123330.31003-4-jani.nikula@intel.com