drm/i915: Kill the remaining CHV HBR2 leftovers

AFAIK CHV was supposed to have HBR2 originally, but in the end the feature
was dropped. We still have some code leftovers from those early days.
Eliminate them.

The extra bit for the training pattern seems to be dead in the hardware.
I can set it (in fact I can set almost any reserved bit in the
registers) but it doesn't seem to interfere with the operation of the
hardware. Either that or I'm very lucky that my displays complete link
training with the incorrect pattern being sent out.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180302095656.19662-1-ville.syrjala@linux.intel.com
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>