st/mesa: move around some code in st_context.c

st/mesa: move around some code in st_context.c

st_init_driver_functions() is only called in st_context.c so there's
no need for the prototype in st_context.h

To avoid a forward declaration of st_init_driver_functions() in
st_context.c, we need to move around several other functions.

No functional change.

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