drm/i915: Drop the drm_atomic_helper_calc_timestamping_constants() call

We update the timestamping constants per-crtc explicitly in
intel_crtc_update_active_timings(). Furtermore the helper will
use uapi.adjusted_mode whereas we want hw.adjusted_mode. Thus
let's drop the helper call an rely on what we already have in
intel_crtc_update_active_timings(). We can now also drop the
hw.adjusted_mode -> uapi.adjusted_mode copy hack that was added
to keep the helper from deriving the timestamping constants from
the wrong thing.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200907120026.6360-3-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>