proptest: Use util_open()

Use the new util_open() helper instead of open-coding the method for
finding a usable device. While at it, make the command-line interface
more consistent with that of modetest by adding the -D and -M options.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>