x86/mm: Cleanup pgprot_4k_2_large() and pgprot_large_2_4k()

Make use of lower level helpers that operate on the raw protection
values to make the code a little easier to understand, and to also
avoid extra conversions in a few callers.

[ Qian: Fix a wrongly placed bracket in the original submission.
  Reported and fixed by Qian Cai <cai@lca.pw>. Details in second
  Link: below. ]

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200408152745.1565832-4-hch@lst.de
Link: https://lkml.kernel.org/r/1ED37D02-125F-4919-861A-371981581D9E@lca.pw