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.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>