drm/i915/gt: Optimise aliasing-ppgtt allocations

Since the aliasing-ppgtt remains the default for gen6/gen7, it is worth
optimising the ppgtt allocation for it. In this case, we do not need to
flush the GGTT page directories entries as they are fixed during setup.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200709190111.5492-1-chris@chris-wilson.co.uk