drm/ttm: remove overlapping memcpy support

remove the overlapping memcp support as it's never used.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201022031152.1916925-1-airlied@gmail.com