The current construction of the named mode parsing doesn't allow to extend
it easily. Let's move it to a separate function so we can add more
parameters and modes.

In order for the tests to still pass, some extra checks are needed, so
it's not a 1:1 move.
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: Mateusz Kwiatkowski <kfyatek+publicgit@gmail.com>
Link: https://lore.kernel.org/r/20220728-rpi-analog-tv-properties-v9-9-24b168e5bcd5@cerno.techSigned-off-by: Maxime Ripard <maxime@cerno.tech>