drm/i915: Flag any possible writes for a GTT fault

We do not explicitly mark the PTE for the user's GTT mmap as being
wrprotect, so we don't get a refault when we would need to change a
read-only mmapping into read-write. As such, we must presume that if the
vma has PROT_WRITE it may be written to, although this is supposed to be
indicated by set-domain there are cases (e.g. after swap) where
userspace may not be aware of the implicit domain change.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180903083337.13134-2-chris@chris-wilson.co.uk