drm/vc4: vec: Remove empty mode_fixup

The mode_fixup hooks are deprecated, and the behaviour we implement is the
default one anyway. Let's remove it.

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-v2-24-459522d653a7@cerno.tech