drm/modes: Rewrite the command line parser

Rewrite the command line parser in order to get away from the state machine
parsing the video mode lines.

Hopefully, this will allow to extend it more easily to support named modes
and / or properties set directly on the command line.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e32cd4009153b184103554009135c7bf7c9975d7.1560783090.git-series.maxime.ripard@bootlin.com