drm/i915: Call i915_perf_fini() on init_hw error unwind

We have to cleanup after i915_perf_init(), even on the error path, as it
passes a pointer into the module to the sysfs core. If we fail to
unregister the sysctl table, we leave a dangling pointer which then may
explode anytime later.

Fixes: 9f9b2792 ("drm/i915/perf: reuse timestamp frequency from device info")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180414091233.32224-1-chris@chris-wilson.co.uk