gbm/dri: Check dri extension version before flush after unmap

gbm/dri: Check dri extension version before flush after unmap

The commit mentioned below required the __DRI2FlushExtension to have
version 4 or above, for GBM functionality. That broke GBM with some
classic dri drivers. Relax that requirement so that we only flush
after unmap if we have version 4 or above. Drivers that require the flush
for correct functionality should implement the desired version.

Fixes: ba8df228 ("gbm/dri: Flush after unmap")
Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Dylan Baker <dylan@pnwbakers.com>