drm/client: Add some tests for drm_connector_pick_cmdline_mode()

drm_connector_pick_cmdline_mode() is in charge of finding a proper
drm_display_mode from the definition we got in the video= command line
argument.

Let's add some unit tests to make sure we're not getting any regressions
there.

Acked-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-v9-8-24b168e5bcd5@cerno.tech


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