st/egl: Reduce validation round-trips in DRI2 backend.

st/egl: Reduce validation round-trips in DRI2 backend.

This is similar to the changes to reduce the round-trips in ximage
backend.

When the server supports DRI_InvalidateBuffers, the event is used to
update the server stamp (this path is not tested).  Otherwise, the
server stamp is updated when flush_frontbuffer or swap_buffers is
called.  This makes sure at least the client APIs get the new buffers
when a new frame is started while skipping all round-trips during the
drawing.