drm/ttm: remove special handling for non GEM drivers

vmwgfx is the only driver actually using this. Move the handling into
the driver instead.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210419092853.1605-1-christian.koenig@amd.com