drm/vc4: vec: Convert to atomic helpers

The VC4 VEC driver still uses legacy enable and disable hook
implementation. Let's convert to the atomic variants.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220728-rpi-analog-tv-properties-v2-25-459522d653a7@cerno.tech