drm/i915: Initialize the aliasing ppgtt as part of global gtt

Stuffing this into the context setup code doesn't make a lot of sense.
Also reusing the real ppgtt setup code makes even less sense since the
aliasing ppgtt isn't a real address space. Leaving all that stuff
unitialized will make sure that we catch any abusers promptly.

This is also a prep work to clean up the context->ppgtt link.

v2: Fix up the logic fail, I've fumbled it so badly to completely
disable ppgtt on gen6. Spotted by Ville and Michel. Also move around
the pde write into the gen6 init function, since otherwise it won't
work at all.

v3: Only initialize the aliasing ppgtt when we actually enable it.

Cc: "Thierry, Michel" <michel.thierry@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
[danvet: Squash in fixup from Fengguang Wu.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>