drm/i915: Fix DRAM size reporting for BXT

The BXT DUNIT register tells us the size of each DRAM device
in Gb. We want to report the size of the whole DIMM in GB, so
that it matches how we report it for non-LP platforms.

v2: Deobfuscate the math (Chris)
    s/GB/GBIT/ in the register bit definitions (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/20190306203551.24592-6-ville.syrjala@linux.intel.com