drm/vc4: 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.

Additionally, remove the superfluous tv_mode field, which was redundant
with the mode field in struct drm_tv_connector_state.

Signed-off-by: Mateusz Kwiatkowski <kfyatek+publicgit@gmail.com>