[POWERPC] spufs: reorganize spu_run_init

This cleans up spu_run_init so that it does all of the spu
initialization for spufs_run_spu.  It initializes the spu context as
much as possible before it activates the spu and writes the runcntl
register.

Signed-off-by: Luke Browning <lukebr@linux.vnet.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>