drm/atomic-helper: Add an analog TV atomic_check implementation

The analog TV connector drivers share some atomic_check logic, and the new
TV standard property have created some boilerplate that can be shared
across drivers too.

Let's create an atomic_check helper for those use cases.

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-v10-14-256dad125326@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>