drm/ttm: rename ttm_resource_manager_func callbacks

The names get/put are associated with reference counting
in the Linux kernel, use alloc/free instead.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/384340/?series=80346&rev=1