When making current, assign the API state's new context and dispatch table
in GLdispatch rather than GLX.  This avoids consistency issues if the API state
is unchanged but the current dispatch or context may have changed.

Update the __glDispatchMakeCurrent() call in
__glXMakeGLDispatchCurrent() to get libglvnd building for now; a later change
will remove libglxgldispatch.c altogether.

Signed-off-by: Brian Nguyen <brnguyen@nvidia.com>