st/mesa: move utility functions, macros into new st_util.h file

st/mesa: move utility functions, macros into new st_util.h file

To de-clutter st_context.h.

Clean up remaining function prototypes in st_context.h.

The st_vp_uses_current_values() helper is only used in st_context.c
so move it there.

The st_get_active_states() function is only used in st_context.c so
remove its prototype in st_context.h

Reviewed-by: Neha Bhende <bhenden@vmware.com>