powerpc/mm: Write to PTCR only if ultravisor disabled

In ultravisor enabled systems, PTCR becomes ultravisor privileged only
for writing and an attempt to write to it will cause a Hypervisor
Emulation Assitance interrupt.

This patch uses the set_ptcr_when_no_uv() function to restrict PTCR
writing to only when ultravisor is disabled.

Signed-off-by: Claudio Carvalho <cclaudio@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190822034838.27876-6-cclaudio@linux.ibm.com