KVM: PPC: Book3S HV P9: Move SPRG restore to restore_p9_host_os_sprs

Move the SPR update into its relevant helper function. This will
help with SPR scheduling improvements in later changes.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211123095231.1036501-19-npiggin@gmail.com