drm/amdgpu: Fix leak when GPU memory allocation fails

Free the sync object if the memory allocation fails for any
reason.

Signed-off-by: Mukul Joshi <mukul.joshi@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>