radeon: Add DRI2 flush extension to so we synchronize properly.

radeon: Add DRI2 flush extension to so we synchronize properly.

When DRI2 swap buffer is pending (copy buffer not pageflipping)
we need to make sure we have the flush extension so radeon doesn't
resume rendering on the not yet blitted front buffer.

Modified version of Jerome's patch to add flush extension
in the correct place.

This prepares a possible fix for:

https://bugs.freedesktop.org/show_bug.cgi?id=28341
https://bugs.freedesktop.org/show_bug.cgi?id=28410

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>