drm/i915: Use dram_dimm_info more

Reduce the code duplication a bit by sharing the same
code for parsing both DIMMs on a channel.

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/20190306203551.24592-8-ville.syrjala@linux.intel.com