drm/ttm: make some inline helper functions for cleanup paths. (v2)

The disable path is just temporary for now, it will be dropped once has_type
is gone in a later patch.

v2: add docs.
rename to ttm_mem_type_manager namespace

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-26-airlied@gmail.com