dri_util: Assume error checking is done properly in glXMakeCurrent

dri_util: Assume error checking is done properly in glXMakeCurrent

In short what the code did before:

    __DRIscreen *psp = NULL;
    if (pcp)
       psp = pcp->psb;

    assert(psp);
    if (psp->stuff)
       other_stuff();

    return psb->even_more(pcp);

Remove all that stupid checking which still segfaults/asserts later on and
just do what we do in driUnbindContext. Also limited testing show libGL never
call driUnbindContext or driBindContext with cPriv == NULL.