powerpc/64s: Fold update_current_thread_[i]amr() into their only callers

lkp reported warnings in some configuration due to
update_current_thread_amr() being unused:

  arch/powerpc/mm/book3s64/pkeys.c:284:20: error: unused function 'update_current_thread_amr'
  static inline void update_current_thread_amr(u64 value)

Which is because it's only use is inside an ifdef. We could move it
inside the ifdef, but it's a single line function and only has one
caller, so just fold it in.

Similarly update_current_thread_iamr() is small and only called once,
so fold it in also.

Fixes: 48a8ab4e ("powerpc/book3s64/pkeys: Don't update SPRN_AMR when in kernel mode.")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210314093320.132331-1-mpe@ellerman.id.au