dri_util: Don't assume __DRIcontext->driverPrivate is a gl_context

dri_util: Don't assume __DRIcontext->driverPrivate is a gl_context

The driverPrivate pointer is opaque to the driver and we can't assume
it's a struct gl_context in dri_util.c.  Instead provide a helper function
to set the struct gl_context flags from the incoming DRI context flags.

v2 (idr): Modify the other classic drivers to also use
driContextSetFlags.  I ran all the piglit GLX_ARB_create_context tests
with i965 and classic swrast without regressions.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> [v1]
Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Ilia Mirkin <imirkin@alum.mit.edu> [v1 on Gallium nouveau]
Cc: "10.0" <mesa-stable@lists.freedesktop.org>