drm/i915: Initialize CHV digital lock detect threshold

Initialize lock detect threshold and select coarse threshold for the
case where M2 fraction division is disabled.

v2: Split the changes into multiple smaller patches (Ville)
v3: Clear out the old bits before we modify those bits as RMW (Ville)
v4: Reset coarse threshold when M2 fraction is enabled (Ville)

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