drm: Add missing break in the command line mode parsing code

As we parse the string given on the command line one char at a time, it
seems that we do want a break at every case.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>