drm/radeon: check return value of radeon_fence_emit

Function radeon_fence_emit() returns -ENOMEM if there is no enough
memory. And in this case, function radeon_ring_unlock_undo() rather than
function radeon_ring_unlock_commit() should be called. However, in
function radeon_test_create_and_emit_fence(), the return value of
radeon_fence_emit() is ignored. This patch adds the check.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>