drm/connector: Rename legacy TV property

The current tv_mode has driver-specific values that don't allow to
easily share code using it, either at the userspace or kernel level.

Since we're going to introduce a new, generic, property that fit the
same purpose, let's rename this one to legacy_tv_mode to make it
obvious we should move away from it.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com> # nouveau
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: Mateusz Kwiatkowski <kfyatek+publicgit@gmail.com>
Acked-in-principle-or-something-like-that-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20220728-rpi-analog-tv-properties-v10-2-256dad125326@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>