drm/i915: Use unchecked writes for setting up the fences

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