commit f6ff4f67 upstream.

An arbitrary amount of time can pass between spin_unlock and
radeon_fence_wait_any, so we need to ensure that nobody frees the
fences from under us.

Based on the analogous fix for amdgpu.
Signed-off-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com> (v1 + fix)
Tested-by: Lutz Euler <lutz.euler@freenet.de>
[ nicolai:
 Backport of upstream commit f6ff4f67, with
 an additional NULL pointer guard that is required for kernels 3.17 and older.

 To be precise, any kernel that does *not* have commit 954605ca "drm/radeon:
 use common fence implementation for fences, v4" requires this additional
 NULL pointer guard.]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>