drm/i915: Don't check for NEEDS_GTT when deciding the address space

This means something different and is only relevant for gen6 and the
reason why we cant use anything else than aliasing ppgtt there.

Note that the currently implemented logic for secure batches is
broken: Userspace wants the buffer both in ppgtt (for self-referencing
relocations) and in ggtt (for priveledge operations).

This is the same issue the command parser is also facing.
Unfortunately our coverage for corner-cases of self-referencing
batches is spotty.

Note that this will break vsync'ed Xv and DRI2 copies.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>