drm/amdgpu: fix leaking fence in the pageflip code

This fixes a memory leak when we can't register the callback on a fence.

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