drm/i915: Read DIMM size in Gb rather than GB

CNL+ can report DIMM sizes in .5 GB units. In order to not trauncate
away the .5 GB let's switch to storing the DIMM size in Gb units.

Cc: Swati Sharma <swati2.sharma@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200929131312.12999-1-ville.syrjala@linux.intel.com