drm/ttm: drop list of memory managers from device. (v2)

The driver now controls these, the core just controls the system
memory one.

v2: init sysman explicitly and assign it as a driver manager
to simplify the lookup sequence.

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/20200804025632.3868079-55-airlied@gmail.com