dri2: Implement a throttle dri extension.

dri2: Implement a throttle dri extension.

The X server has limited throttle support on the server side,
but doing this in the client has some benefits:

1) X server throttling is per client. Client side throttling can be done
per drawable.

2) It's easier to control the throttling based on what client is run,
for example using "driconf".

3) X server throttling requires drm swap complete events.

So implement a dri2 throttling extension intended to be used by direct
rendering clients.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Reviewed-by: Michel Dänzer <michel@daenzer.net>