drm/i915: Remove identical write mmmio functions

We notice two identical copies of the shadow register table and
following from that removal can also unify CHV and Gen9 write
mmio functions and macros into a single implementation.

v2: Name fwtable consistently and use HAS_FWTABLE. (Joonas Lahtinen)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>