radeon: Add DRI2 flush extension support, so we synchronize properly.

radeon: Add DRI2 flush extension support, so we synchronize properly.

When a DRI2 swap buffer is pending we need to make sure we
have the flush extension so radeon doesn't resume rendering to
or reading from the not yet blitted front buffer.

This fixes:

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>