intel: use throttle ioctl for throttling

intel: use throttle ioctl for throttling

Rather than waiting on the first batch after the last swapbuffers to be
retired, call into the kernel to wait upon the retirement of any request
less than 20ms old. This has the twofold advantage of (a) not blocking
any other clients from utilizing the device whilst we wait and (b) we
attain higher throughput without overloading the system.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>