Subplatform, stepping and cleanup

First 2 commits are to be merged/viewed in !63 (merged), which add the PCI IDs in a new header.

This MR adds support for both subplatform, with DG2 as the first user, and stepping information. The former is greatly simplified from what there is in i915, tying the subplatform to the device info. The latter is mostly borrowed from i915 with renames and few refactors.

Signed-off-by: Lucas De Marchi lucas.demarchi@intel.com