drm/selftests: modes: Add more unit tests for the cmdline parser

Let's add some unit tests for the recent bugs we just fixed.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Tested-by: Thomas Graichen <thomas.graichen@gmail.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190827115850.25731-4-mripard@kernel.org