i915/perf: Fix NULL deref bugs with drm_dbg() calls
In the Linux kernel, the following vulnerability has been resolved:

i915/perf: Fix NULL deref bugs with drm_dbg() calls

When i915 perf interface is not available dereferencing it will lead to
NULL dereferences.

As returning -ENOTSUPP is pretty clear return when perf interface is not
available.

[tursulin: added stable tag]
(cherry picked from commit 36f27350ff745bd228ab04d7845dfbffc177a889)