dri glx: Fix dri_util::driBindContext

dri glx: Fix dri_util::driBindContext

1) Don't error-check here. It's done in glx makeCurrent.
2) Allow ctx and the dri drawables to be NULL for future use. This is
   currently blocked in glx makeCurrent.
3) Avoid updating dri drawables unless they are completely uninitialized.
   Since the updating was done outside of the lock, the driver need to
   verify and redo it anyway.

Signed-off-by: Thomas Hellstrom <thellstrom-at-vmware-dot-com>