drm/i915: Extract DIMM info on cnl+

We'll need information about the memory configuration on cnl+ too.
Extend the code to parse the slightly changed register layout.

v2: Document what cnl_get_dimm_size() returns (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/20190306203551.24592-12-ville.syrjala@linux.intel.com