mesa,i965: use NewDriverState to communicate TFB state changes with the driver

mesa,i965: use NewDriverState to communicate TFB state changes with the driver

_NEW_TRANSFORM_FEEDBACK is not used by core Mesa, so it can be removed.
Instead, an new private flag is added to i965 to serve the same purpose.

If you're new to this:

* When creating a context. you can set private dirty flags
  in gl_context::DriverFlags, eg.:
    ctx->DriverFlags.NewStateX = BRW_NEW_STATE_X;

* When StateX is changed, core Mesa does:
    ctx->NewDriverState |= ctx->DriverFlags.NewStateX;

* When you have to draw, read and clear ctx->NewDriverState.

* Pros: not touching NewState, the driver decides the mapping between
  GL states and hw state groups, unlimited number of flags in core Mesa
  (still limited number of flags in the driver though)

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>