drm/i915/bxt: Fix GRC code register field definitions

This has been corrected in BSpec quite some time ago, but we missed it
somehow. The wrong field definitions resulted in configuring PHY0 with
an incorrect GRC value.

v2:
- Remove the FIXME comment, we left in the code exactly about this
  issue. (Ville)

CC: Arthur J Runyan <arthur.j.runyan@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459515767-29228-3-git-send-email-imre.deak@intel.com