drm/i915: Polish skl_is_16gb_dimm()

Pass the dimm struct to skl_is_16gb_dimm() rather than passing each
value separately. And let's replace the hardcoded set of values with
some simple arithmetic.

Also fix the byte vs. bit inconsistency in the debug message,
and polish the wording otherwise as well.

v2: Deobfuscate the math (Chris)

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/20190306203551.24592-4-ville.syrjala@linux.intel.com