drm/modes: Introduce a whitelist for the named modes

The named modes support has introduced a number of glitches that were in
part due to the fact that the parser will take any string as a named mode.

Since we shouldn't have a lot of options there (and they should be pretty
standard), let's introduce a whitelist of the available named modes so that
the kernel can differentiate between a poorly formed command line and a
named mode.

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/20190827115850.25731-3-mripard@kernel.org