mm: ptep_deref() conversion

Convert all instances of direct pte_t* dereferencing to instead use
ptep_deref() helper. By default, the helper does a direct dereference as
before, but it can (and will) be overridden by the architecture to fully
encapsulate the contents of the pte. Arch code is deliberately not
converted, as the arch code knows best.

Conversion was done using Coccinelle:

----

// $ make coccicheck \
//          COCCI=ptepderef.cocci \
//          SPFLAGS="--include-headers" \
//          MODE=patch

virtual patch

@ depends on patch @
pte_t *v;
@@

- *v
+ ptep_deref(v)

----

Then reviewed and hand-edited to avoid multiple unnecessary calls to
ptep_deref(), instead opting to store the result of a single in a
variable, where it is correct to do so. This will benefit arch-overrides
that may be more complex than a simple (optimizable) pointer
dereference.

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