drm/i915: Update DDL only for current CRTC

Instead of looping through all CRTCs, update DDL for current CRTC for which
watermark is being updated.
CHV is confirmed to have precision of 32/64 which is same as VLV.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Gajanan Bhat <gajanan.bhat@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>