drm/i915/bxt: Use revid->stepping tables

Switch BXT to use a revid->stepping table as we're trying to do on all
platforms going forward.  Note that the REVID macros we had before
weren't being used anywhere in the code and weren't even correct; the
table values come from the bspec (and omits all the placeholder and
preproduction revisions).

Although nothing in the code is using the data from this table at the
moment, we expect some upcoming DMC patches to start utilizing it.

Bspec: 13620
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210713193635.3390052-6-matthew.d.roper@intel.com