mesa: another fix for program/texture state validation

mesa: another fix for program/texture state validation

This fixes a regression introduced in 46ae1abbac6837d051c10b2e8b57eab3d4958ff4

Break program validation into two steps, do part before texture state
validation and do the rest after:
1. Determine Vertex/Fragment _Enabled state.
2. Update texture state.
3. Determine pointers to current Vertex/Fragment programs (which may involve
generating new "fixed-function" programs).

See comments in the code for more details of the dependencies.