drm/i915/bxt: Add a note about BXT_PORT_CL1CM_DW30 being read-only

This register is read-only, so we have never actually set
OCL2_LDOFUSE_PWR_DIS in it as specified by the specification. Add a code
comment about this. I filed a specification update request to clarify
this there.

CC: Arthur J Runyan <arthur.j.runyan@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: David Weinehall <david.weinehall@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459515767-29228-4-git-send-email-imre.deak@intel.com