glx: Don't allow glXMakeContextCurrent() with only one valid drawable

Drawable and readable need to either both be None or both be non-None.

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit d257ec01)