drm/vc4: Refactor mode checking logic

Replace drm_encoder_helper_funcs::atomic_check with
drm_connector_helper_funcs::atomic_check - the former is not called
during drm_mode_obj_set_property_ioctl(). Set crtc_state->mode_changed
if TV norm changes even without explicit mode change. This makes things
like "xrandr --output Composite-1 --set mode PAL-M" work properly.

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