drm/i915: Drop dead code for pvc

PCI IDs for PVC were never added and platform always marked with
force_probe. Drop what's not used and rename some places as needed.

The registers not used anymore are also removed.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20240320060543.4034215-6-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>