loader/dri3: Use client local back to front blit in copySubBuffer if available

loader/dri3: Use client local back to front blit in copySubBuffer if available

The copySubBuffer functionality always attempted a server side blit from
back to fake front if a fake front was present, and we weren't displaying
on a remote GPU.

Now that we always have local blit capability on modern drivers, first
attempt a local blit, and only if that fails, try the server blit.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Axel Davy <axel.davy@normalesup.org>