drm/atomic-helpers: Update vblank timestamping constants

The atomic helpers don't call drm_calc_timestamping_constants, which
is a regression compared to the crtc helpers. Fix this.

Noticed while reviewing i915 atomic patches from Maarten.

v2: Also check state->enable to avoid a warning in dmesg. Reported by
Maarten.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>