drm/i915: Extract functions to derive SKL+ DIMM info

Make the code less repetitive by extracting a few small helpers.

v2: Squash in the switch removal for skl_get_dimm_ranks()
    (it got misplaced in a rebase accident)
    Document what skl_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/20190306203551.24592-3-ville.syrjala@linux.intel.com