drm/cmdline-parser: Use assert when needed

Expecting to observe a specific value, when the function responsible for
setting the value has failed will lead to extra noise in test output.
Use assert when the situation calls for it.
Also - very small tidying up around the changed areas (whitespace).

v2: Leave out the locals (drm_connector is huge) (lkp)
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20220817211236.252091-2-michal.winiarski@intel.com