drm/i915/tgl: Use TGL stepping info for applying WAs

TGL adds another level of indirection for applying WA based on stepping
information rather than PCI REVID. So change TGL_REVID enum into
stepping enum and use PCI REVID as index into revid to stepping table to
fetch correct display and GT stepping for application of WAs as
suggested by Matt Roper.

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Aditya Swarup <aditya.swarup@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210119192931.1116500-1-lucas.demarchi@intel.com