drm/amdgpu: fix mmhub register base coding error

fix MMHUB register base coding error.

Fixes: ec683759 ("drm/amdgpu/gmc10: program the smallK fragment size")

Signed-off-by: Yang Wang <KevinYang.Wang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org