drm/i915/uncore: Replace gen8 write functions with general fwtable

Now that we have both a standard forcewake table (albeit a single-entry
table) and the shadow table stored in the uncore, we can drop the
gen8-specific write handlers in favor of the general fwtable version.

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/20210910201030.3436066-4-matthew.d.roper@intel.com