drm/radeon: check the right ring in radeon_evict_flags()

commit 5e5c21cac1001089007260c48b0c89ebaace0e71 upstream.

Check the that ring we are using for copies is functional
rather than the GFX ring.  On newer asics we use the DMA
ring for bo moves.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>