drm/i915/uncore: Associate shadow table with uncore

Store a reference to a platform's shadow table inside the uncore, the
same as we do with the forcewake table.  This will allow us to use a
single set of functions that operate on the shadow table reference
rather than generating lots of nearly-identical functions via macros
that differ only in terms of the table that they reference.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210910201030.3436066-3-matthew.d.roper@intel.com