drm/i915: Don't look at pg_dirty_rings for aliasing ppgtt

We load the ppgtt ptes once per gpu reset/driver load/resume and
that's all that's needed. Note that this only blows up when we're
using the allocate_va_range funcs and not the special-purpose ones
used. With this change we can get rid of that duplication.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>