drm/selftests: Add command line parser selftests

The command line parser is pretty tough to get right and very error prone,
so let's add a selftest to try to catch any regression.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/735348d3a475ce0bcc88b46e093ab149023bb202.1560783090.git-series.maxime.ripard@bootlin.com