drm/i915: Eliminate crtc->state usage from intel_atomic_commit_tail and .crtc_update()

We already have the correct new crtc state so just use that instead of
crtc->state.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170823152226.22938-7-ville.syrjala@linux.intel.com