drm/vc4: Always obey implicit sync

Same justification as for drm_gem_fb_prepare_fb.

Cc: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180405154449.23038-9-daniel.vetter@ffwll.ch
Link: https://patchwork.freedesktop.org/patch/msgid/20180409085134.27321-1-daniel.vetter@ffwll.ch