glx: Only flush indirect contexts in MakeCurrent (v2)

If the context is direct none of the GL commands were issued by this
process, the server couldn't flush them even if it wanted to.

v2: Fix embarassingly obvious boolean inversion (Michel Dänzer)

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
(cherry picked from commit 307c124d)