drm/radeon: cleanup ttm operation ctx usage.

Just pass it around move, and remove unused pieces

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200923030454.362731-4-airlied@gmail.com