glx/drisw: Fix glXMakeCurrent(dpy, None, ctx)

This is perfectly legal in GL 3.0+.

Fixes piglit/glx-create-context-current-no-framebuffer.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 033cfb17)