drm/i915: Convert icl_get_stolen_reserved to uncore mmio accessors

More removal of implicit dev_priv.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190610120608.15477-3-tvrtko.ursulin@linux.intel.com