The legacy DRM API adds some complexity to the DRM code. There
are only 4 drivers that do not support the DRM Atomic API:
1. radeon (early GCN amd cards)
2. gma500 (ancient intel GPUs)
3. ast (ASPEED SoCs)
4. nouveau
Going forward, new DRM drivers will be guaranteed to support the atomic
API so this is a safe removal.