loader_dri3: Eliminate the back-to-fake-front copy

loader_dri3: Eliminate the back-to-fake-front copy

Eliminate the back-to-fake-front copy by exchanging the previous back buffer
and the fake front buffer. This is a gain except when we need to preserve
the back buffer content but in that case we still typically gain by replacing
a server-side blit by a client side non-flushing blit.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>