drm: disallow legacy dma ioctls for modesetting drivers

Now only legacy ums drivers have the DRIVER_HAVE_DMA driver feature
flag set, so strictly speaking the modesetting check is redundant. But
adding it has the upside that it makes it very clear that the dma
support is legacy stuff.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>