drm/amdgpu: drop copy/paste leftover to fix big endian

The buf swap field doesn't exist on RB1.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>