drm/i915: switch intel_uncore_forcewake_for_reg to intel_uncore

The intel_uncore structure is the owner of FW, so subclass the
function to it.

While at it, use a local uncore var and switch to the new read/write
functions where it makes sense.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190325214940.23632-7-daniele.ceraolospurio@intel.com