samples: drivers: video: fix board definitions

By having an "auto-fallback" mechanism, the VIDEO_SW_GENERATOR did shadow
the fact that "platform:mimxrt1064_evk:SHIELD=dvp_fpc24_mt9m114" was
missing the full specification, and therefore was not matched at all:
the shield was not selected, but the CI still worked.

Signed-off-by: Josuah Demangeon <me@josuah.net>