drm/radeon: Skip dma copy test in benchmark if card doesn't have dma engine.

radeon_copy_dma is only available for r200 or newer cards.
Call to radeon_copy_dma would result to NULL pointer
dereference if benchmarking asic without dma engine.

Signed-off-by: Pauli Nieminen <suokkos@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>