drm/i915: Add new PHY reg definitions for lock threshold

Added new PHY register definitions to control TDC buffer calibration and
digital lock threshold.

Signed-off-by: Vijay Purushothaman <vijay.a.purushothaman@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>