drm/tidss: Add prepare_fb to the plane helper funcs

drm_gem_fb_prepare_fb() extracts fence and attaches to plane state.
The fence info is needed if implicit fencing is used. Add this as
prepare_fb function pointer to plane helper funcs.

Signed-off-by: Gowtham Tammana <g-tammana@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826134409.473554-1-tomi.valkeinen@ti.com