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

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>
Cc: stable@vger.kernel.org