drm/i915: move add_taint_for_CI() to i915_utils.h

Seems like a better fit. Reduce clutter in i915_drv.h.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a82d8c4e95496b3b4a9a251b655ea56a35c9d752.1565271681.git.jani.nikula@intel.com