mesa: Replace use of _ReallyEnabled as a boolean with use of _Current.

mesa: Replace use of _ReallyEnabled as a boolean with use of _Current.

I'm probably not the only person that has tried to kill _ReallyEnabled.
This does the mechanical part of the work, and cleans _ReallyEnabled from
i965.

I think that using _Current makes texture management clearer: You can't
have multiple targets in use in the same texture image unit at the same
time, because there's just that one pointer.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>