drm/i915/execlists: Move RCS mmio workaround to new common wa_list

We can move the remaining RCS workarounds applied to only gen8 to the
engine->wa_list, and then reduce all engine->init_hw callbacks to common
code. The benefit of using the new wa_list is that we verify that the
registers are indeed restored and keep their magic values.

v2: INSTPM_FORCE_ORDERING is already part of gen8_ctx_workarounds, and
as confirmed by the mmio verification is a part of the context image!
v3: MI_MODE is already part of gen8_ctx_workarounds...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181206180713.6827-2-chris@chris-wilson.co.uk