As part of the command line parsing rework coming in the next patches,
we'll need to lookup drm_connector_tv_mode values by their name, already
defined in drm_tv_mode_enum_list.

In order to avoid any code duplication, let's do a function that will
perform a lookup of a TV mode name and return its value.
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-v10-7-256dad125326@cerno.techSigned-off-by: Maxime Ripard <maxime@cerno.tech>