[POWERPC] fix PMU initialization on pseries lpar

We should not be calling power4_enable_pmcs() in
pseries_lpar_enable_pmcs(); just doing the hypercall is sufficient.
Prior to 2.6.15 we did not call power4_enable_pmcs() for an lpar.

power4_enable_pmcs() tries to read the hid0 register which is no
longer legal for an lpar in newer Power processors.

Signed-off-by: Paul Mackerras <paulus@samba.org>