drm/i915/perf: fix oa config reconfiguration

The current logic just reapplies the same configuration already stored
into stream->oa_config instead of the newly selected one.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 7831e9a9 ("drm/i915/perf: Allow dynamic reconfiguration of the OA stream")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191019214647.27866-1-lionel.g.landwerlin@intel.com