drm/i915/uncore: Reorganize and document shadow and forcewake tables

Let's reorganize some of the forcewake/shadow handling in intel_uncore.c
and consolidate the cargo-cult comments on each table into more general
comments that apply to all tables.

We'll probably move forcewake handling to its own dedicated file in the
near future and further enhance this with true kerneldoc.  But this is a
good intermediate step to help clarify the behavior a bit.

Cc: Stuart Summers <stuart.summers@intel.com>
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/20220511060228.1179450-2-matthew.d.roper@intel.com