drm/i915/uc: Gt-fy uc reset

This was the last place in gt/uc that was still using I915_READ
with the global dev_priv.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190723091404.6449-1-chris@chris-wilson.co.uk