video/tmep038.cpp: Use device_gfx_interface for gfx decode, Fix notes

[cam900]

atlus/cave.cpp: Reduce duplicates

[cuavas]

The changes look OK, but is it really a win in this case?

• The 038 tilemap device doesn’t use a fixed tile format, so the actual graphics decoding layout still needs to be supplied by the user.
• It seems to require more configuration this way than it did before.
• If multiple chips use the same tiles, they’ll end up with duplicated copies of the decoded tiles.

Overall, this doesn’t look like a clear improvement.