drm/i915/perf: Avoid polluting the i915_oa_config with error pointers

Use a local variable to track the allocation errors to avoid polluting
the struct and keep the free simple.

Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191013095211.2922-1-chris@chris-wilson.co.uk