BACKPORT: arm64: pgtable: Implement p[mu]d_valid() and check in set_p[mu]d()

Now that our walk-cache invalidation routines imply a DSB before the
invalidation, we no longer need one when we are clearing an entry during
unmap.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Change-Id: Ib0ad415b232f766fb93455f39de5449f4bf45dfb
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
[cyberknight777: Backport to msm-4.14]
Signed-off-by: Cyber Knight <cyberknight755@gmail.com>