drm/atomic-helper: support more than one write fence in drm_gem_plane_helper_prepare_fb

Use dma_resv_get_singleton() here to eventually get more than one write
fence as single fence.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220321135856.1331-13-christian.koenig@amd.com