drm/i915/perf: fix perf enable/disable ioctls with 32bits userspace

The compat callback was missing and triggered failures in 32bits
userspace when enabling/disable the perf stream. We don't require any
particular processing here as these ioctls don't take any argument.
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: eec688e1 ("drm/i915: Add i915 perf infrastructure")
Cc: linux-stable <stable@vger.kernel.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171024152728.4873-1-lionel.g.landwerlin@intel.com

drivers/gpu/drm/i915/i915_perf.c

@@ -2537,6 +2537,10 @@ static const struct file_operations fops = {
 	.poll		= i915_perf_poll,
 	.read		= i915_perf_read,
 	.unlocked_ioctl	= i915_perf_ioctl,
+	/* Our ioctl have no arguments, so it's safe to use the same function
+	 * to handle 32bits compatibility.
+	 */
+	.compat_ioctl   = i915_perf_ioctl,
 };