radeon: Fix VCE ring test for Big-Endian systems

This patch fixes the VCE ring test when running on Big-Endian machines.
Every write to the ring needs to be translated to little-endian.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>