drm/vc4: vec: Refactor VEC TV mode setting

Change the mode_set function pointer logic to declarative config0,
config1 and custom_freq fields, to make TV mode setting logic more
concise and uniform.

Signed-off-by: Mateusz Kwiatkowski <kfyatek+publicgit@gmail.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
[Maxime: Fixed != 0 check, added tv_mode variable]
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220728-rpi-analog-tv-properties-v2-26-459522d653a7@cerno.tech