drm/modes: Skip invalid cmdline mode

The named mode could be invalid and then cmdline parser misses to validate
mode's dimensions, happily adding 0x0 mode as a valid mode. One case where
this happens is NVIDIA Tegra devices that are using downstream bootloader
which adds "video=tegrafb" to the kernel's cmdline and thus upstream Tegra
DRM driver fails to probe because of the invalid mode.

Fixes: 3aeeb13d

 ("drm/modes: Support modes names on the command line")
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190709145151.23086-1-digetx@gmail.com