drm/i915: Make sure hardware uses the correct swing margin/deemph bits on chv

The register can house two different swing marging/deemph settings at
once. However only one gets used based on some other bits. Make sure we
set those bits correctly to make the hardware use the settings we
provided.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>