drm/i915: abstract virtual PCH id detection

Make the code slightly more pleasant to look at. 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/38ee1ac06c6724e888679eb287af36c221bd399b.1517851783.git.jani.nikula@intel.com