drm/i915: Store DIMM rank information as a number

Life will be easier later if we have the ranks stored
as a bare number.

v2: s/%d/%u/ all over (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190306203551.24592-2-ville.syrjala@linux.intel.com