radeon/cik: Fix GFX IB test on Big-Endian

This patch makes the IB test on the GFX ring pass for CI-based cards
installed in Big-Endian machines.

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>