drm/modes: Introduce the tv_mode property as a command-line option

Our new tv mode option allows to specify the TV mode from a property.
However, it can still be useful, for example to avoid any boot time
artifact, to set that property directly from the kernel command line.

Let's add some code to allow it, and some unit tests to exercise that code.

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-v10-8-256dad125326@cerno.tech


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