drm_gem_plane_helper_prepare_fb() was using
drm_atomic_set_fence_for_plane() which ignores all implicit fences when an
explicit fence is already set. That's rather unfortunate when the fb still
has a kernel fence we need to wait for to avoid presenting garbage on the
screen.

So instead update the fence in the plane state directly. While at it also
take care of all potential GEM objects and not just the first one.

Also remove the now unused drm_atomic_set_fence_for_plane() function, new
drivers should probably use the atomic helpers directly.

v2: improve kerneldoc, use local variable and num_planes, WARN_ON_ONCE
    on missing planes.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v1)
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220429134230.24334-1-christian.koenig@amd.com