drm/ast: Do not call drm_atomic_add_affected_planes()

There's no need to add planes to the atomic state. Remove the call
to drm_atomic_add_affected_planes() from ast.

On full modesets, the DRM helpers already add a CRTC's planes to the
atomic state; see drm_atomic_helper_check_modeset(). There's no reason
to call drm_atomic_add_affected_planes() unconditionally in the CRTC's
atomic_check() in ast. It's also too late, as the atomic_check() of
the added planes will not be called before the commit.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221013112923.769-4-tzimmermann@suse.de