mm: Move ptep_get() and pmdp_get() helpers

There are many call sites that directly dereference a pte_t pointer.
This makes it very difficult to properly encapsulate a page table in the
arch code without having to allocate shadow page tables.

We will shortly solve this by replacing all the call sites with
ptep_get() calls. But there are call sites above the function definition
in the header file, so let's move ptep_get() to an earlier location to
solve that problem. And move pmdp_get() at the same time to keep it
close to ptep_get().

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>