drm/i915: Request PIN_GLOBAL when pinning a vma for GTT relocations

Always require PIN_GLOBAL when we want a mappable offset (PIN_MAPPABLE).
This causes the pin to fixup the global binding in cases were the vma
was already bound (and due to the proceeding bug, we considered it to be
already mappable).

References: https://bugs.freedesktop.org/show_bug.cgi?id=85671
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Add WARN_ON to check that PIN_MAP implies PIN_GLOBAL as
discussed on irc.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>