drm/radeon: Add asic hook for dma copy to r200 cards.

r200 cards have dma engine which can be used to tranfer data
between vram and system memory.

r300 dma engine registers match r200 dma engine. Enabling
dma copy for r200 is simple as hooking r200 asic to already
existing function r300_copy_dma.

Rename r300_dma_copy to r200_dma_copyto reflect that supports
starts from r200 cards.

v2: Created a new asic object for r200 cards.

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