drm/i915/pvc: Extract stepping information from PCI revid

For PVC, the base die and compute tile have separate stepping values
that we need to track; we'll use the existing graphics_step field to
represent the compute tile stepping and add a new 'basedie_step' field.

Unlike past platforms, steppings for these components are represented by
specific bitfields within the PCI revision ID, and we shouldn't make
assumptions about the non-CT, non-BD bits staying 0.  Let's update our
stepping code accordingly.

Bspec: 44484
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Atwood <matthew.s.atwood@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220527163348.1936146-2-matthew.d.roper@intel.com