egl: Restore "bogus" DRI2 invalidate event code.

egl: Restore "bogus" DRI2 invalidate event code.

I had removed it in commit 1e7776ca2bc59a6978d9b933d23852d47078dfa8
because it was obviously wrong -- why do we care whether the server is a
version that emits events, if we're not watching for the server's events,
anyway?  And why would you only invalidate on a server that emits
invalidate events, when the comment said to emit invalidates if the server
*doesn't*?  Only, I missed that we otherwise don't flag that our buffers
might have changed at swap time at all, so the driver was only checking
for new buffers when triggered by the Viewport hack.  Of course you don't
expect Viewport to be called after a swap.

So, this is effectively a revert of the previous commit, except that I
dropped the check for only emitting invalidates on a new server -- we
*always* need to invalidate if we're doing a SwapBuffers.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=63435
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: "9.1 and 9.2" <mesa-stable@lists.freedesktop.org>