[POWERPC] spufs: update master runcntl with context lock held

Currently, we update the SPU master run control bit (ie,
spu_enable_spu) in spufs_run_spu before we grab the context mutex. This
can result in races with other processes accessing this context's
resources.

This change moves the spu_enable_spu to after we have acquired the
context lock.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>