iris: Do not dereference nullptr with pipe_reference

iris: Do not dereference nullptr with pipe_reference

../src/gallium/drivers/iris/iris_fence.h:54:8: runtime error: member access within null pointer of type 'struct iris_syncpt'

../src/gallium/drivers/iris/iris_fence.c:136:8: runtime error: member access within null pointer of type 'struct pipe_fence_handle'

Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3825>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3825>