drm/sun4i: wait on implicit fence before display

Render like lima will attach a fence to the framebuffer
dma_buf, display like sun4i should wait it finish before
show the framebuffer. Otherwise tearing will be observed.

Signed-off-by: Qiang Yu <yuq825@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181122014417.23285-1-yuq825@gmail.com