drm/atomic-helper: Remove the timestamping constant update from drm_atomic_helper_update_legacy_modeset_state()

The timestamping constants have nothing to do with any legacy state
so should not be updated from
drm_atomic_helper_update_legacy_modeset_state().

Let's make everyone call drm_atomic_helper_calc_timestamping_constants()
directly instead of relying on
drm_atomic_helper_update_legacy_modeset_state() to call it.

@@
expression S;
@@
- drm_atomic_helper_calc_timestamping_constants(S);

@@
expression D, S;
@@
  drm_atomic_helper_update_legacy_modeset_state(D, S);
+ drm_atomic_helper_calc_timestamping_constants(S);

v2: Update drm_crtc_vblank_helper_get_vblank_timestamp{,_internal}() docs (Daniel)
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200907120026.6360-2-ville.syrjala@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>