drm/i915: Extract BXT DIMM helpers

Polish the bxt DIMM parsing by extracting a few small helpers.

v2: Use struct dram_dimm_info
v3: Document what bxt_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/20190306203551.24592-5-ville.syrjala@linux.intel.com