drm/atomic-helper: Extract drm_atomic_helper_calc_timestamping_constants()

Put the vblank timestamping constants update loop into its own
function. It has no business living inside
drm_atomic_helper_update_legacy_modeset_state() so we'll be wanting
to move it out entirely. As a first step we'll still call it
from drm_atomic_helper_update_legacy_modeset_state().

v2: Drop comment about 'legacy state' in the new function

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200907120026.6360-1-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/include/drm/drm_atomic_helper.h b/include/drm/drm_atomic_helper.h
index b268180..85df04c 100644
--- a/include/drm/drm_atomic_helper.h
+++ b/include/drm/drm_atomic_helper.h
@@ -74,6 +74,9 @@ void
 drm_atomic_helper_update_legacy_modeset_state(struct drm_device *dev,
 					      struct drm_atomic_state *old_state);
 
+void
+drm_atomic_helper_calc_timestamping_constants(struct drm_atomic_state *state);
+
 void drm_atomic_helper_commit_modeset_disables(struct drm_device *dev,
 					       struct drm_atomic_state *state);
 void drm_atomic_helper_commit_modeset_enables(struct drm_device *dev,