drm/i915: Fix comments about GMBUSFREQ register

drm/i915: Fix comments about GMBUSFREQ register

The comment about GMBUSFREQ is confused. The spec actually explains
the 4MHz thing perfectly by noting that the 4MHz divider values is
actually just bits [9:2] not [9:0], hence the divide by 1000 correct.
Replace the confused note with a quote from the spec, and eliminate
the duplicated comment that snuck in.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461689194-6079-4-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>