drm/radoen: kill radeon_dma_ioctl_kms

No need to create a dummy ioctl function to return -EINVAL, since
that's what the core already does in the absence of the dma_ioctl
callback. So we can safely remove this.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@gmail.com>