i965: Properly reject __DRI_CTX_FLAG_ROBUST_BUFFER_ACCESS when __DRI2_ROBUSTNESS...

i965: Properly reject __DRI_CTX_FLAG_ROBUST_BUFFER_ACCESS when __DRI2_ROBUSTNESS is not enabled

Only allow __DRI_CTX_FLAG_ROBUST_BUFFER_ACCESS in brwCreateContext if
intelInitScreen2 also enabled __DRI2_ROBUSTNESS (thereby enabling
GLX_ARB_create_context).

This fixes a regression in the piglit test
"glx/GLX_ARB_create_context/invalid flag"

v2: Remove commented debug code.  Noticed by Jordan.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reported-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: "10.0" <mesa-stable@lists.freedesktop.org>