drm/i915: Make PTE valid encoding optional

We need this to work around a corruption when the boot kernel image
loads the hibernated kernel image from swap on Haswell systems -
somehow not everything is properly shut off.

This is just the prep work, the next patch will implement the actual
workaround.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Add a commit message suitable for -fixes and add cc: stable]
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>