As the fence registers are not part of the engine powerwells, we do not
need to fiddle with forcewake in order to update a fence. Avoid using
the heavyweight debug checking normal mmio writes as the checking
dominates the selftest runtime and is superfluous!

In the process, retire the I915_WRITE() implicit macro with the new
intel_uncore_write interface.

v2: s/unc/uncore/
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190604120022.20472-2-chris@chris-wilson.co.uk