drm: remove legacy API

[LaserEyess]

The legacy DRM API adds some complexity to the DRM code. There are only 4 drivers that do not support the DRM 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.

tested on --vo=drm and --vo=gpu{,-next} --gpu-context=drm