drm/amdgpu: fix the build on big endian

Some leftover copy and pastes from radeon that never
got updated.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>