drm/omap: Follow implicit fencing in prepare_fb

I guess no one ever tried running omap together with lima or panfrost,
not even sure that's possible. Anyway for consistency, fix this.

Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Tomi Valkeinen <tomba@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210622165511.3169559-12-daniel.vetter@ffwll.ch