drm/amdgpu: Conform to SET_UCONFIG_REG spec

The packet expects only 16 bits register offset. Hence pass register
offset which is local to each XCC.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>