drm/radeon: finally fix the racy VMA setup

Finally clean up the VMA setup for radeon now that TTM exports the
necessary functions.

No functional change, but only compile tested.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/339354/