glx: Move the GLX variable caching what GL context is current to dix.

glx: Move the GLX variable caching what GL context is current to dix.

GLX is trying to track whether the context it wants is current, to
avoid the glFlush() (and the rest of the overhead) that occurs on all
MakeCurrent calls.  However, its cache can be incorrect now that
glamor exists.  This is a step toward getting glamor to coordinate
with GLX.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>