drm/ttm: rename ttm_mem_type_manager -> ttm_resource_manager.

This name makes a lot more sense, since these are about managing
driver resources rather than just memory ranges.

Acked-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/20200804025632.3868079-59-airlied@gmail.com