drm/atomic_helper: Add missing NULL check for drm_plane_helper_funcs.atomic_update

Something I discovered while writing rvkms since some versions of the
driver didn't have a filled out atomic_update function - we mention that
this callback is "optional", but we don't actually check whether it's NULL
or not before calling it. As a result, we'll segfault if it's not filled
in.

  rvkms rvkms.0: [drm:drm_atomic_helper_commit_modeset_disables] modeset on [ENCODER:36:Virtual-36]
  BUG: kernel NULL pointer dereference, address: 0000000000000000
  PGD 0 P4D 0
  Oops: Oops: 0010 [#1] PREEMPT SMP NOPTI
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS edk2-20240813-1.fc40 08/13/2024
  RIP: 0010:0x0

So, let's fix that.

Signed-off-by: Lyude Paul <lyude@redhat.com>