Fix DMD Gamma Test slide
Fix DMD Gamma Test slide

I tried to use the built in dmd_gamma_test slide and kept crashing with:
`Shutdown reason: BCP client local_display disconnected and exit_on_close is set`

Eventually I copied over the built-in slide code from this repo, and found that it was breaking on `style: dmd_small`:
https://github.com/missionpinball/mpf-mc/blob/efb6bfe5e58826e6545998a0ae9d7108e51ca1e3/mpfmc/mcconfig.yaml#L173

It looks like these styles were recently renamed, and sure enough, replacing `dmd_small` with `small` works great.
missionpinball/mpf-mc,missionpinball/mpf-mc,missionpinball/mpf-mc