sna: Disable the reduced flush optimisation

Deferring the flush until Mesa checks its DRI2 buffer status only works
so long as Mesa is checking its DRI2 buffers. Hint, it doesn't....

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97914
References: https://bugs.freedesktop.org/show_bug.cgi?id=52930
References: https://bugs.freedesktop.org/show_bug.cgi?id=90264
References: https://bugs.freedesktop.org/show_bug.cgi?id=101819
References: https://bugs.freedesktop.org/show_bug.cgi?id=101620


Fixes: 1f6dfc9d ("sna: Only flush GPU bo for a damage event")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>