drm/i915/tv: Use intel_flush_display_plane() to flush the primary plane

Instead of writing to the DSP_ADDR ourselves. This will do the right
thing on gen >= 4 as well.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>