drm/i915: Teach intel_workarounds to use uncore mmio access

Start weaning ourselves off the implicit I915_WRITE macro madness and
start using the explicit intel_uncore mmio access.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190412202458.10653-1-chris@chris-wilson.co.uk