drm: Move legacy buffer structures to <drm/drm_legacy.h>

A few odd cases:
- mgag200 someho had a totally unused drm_dma_handle_t. Remove it.
- i915 still uses the legacy pci dma alloc api, so grows an include.

Everything else fairly standard.

v2: Include "drm_legacy.h" in drm.ko source files for consistency.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>