drm/i915/gt: Set the PP_DIR registers upon enabling ring submission

Always prime the page table registers before starting the ring. Even
though we will update these to the per-context page tables during
dispatch, it is prudent to ensure that the registers always point to a
valid PD.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200206014439.2137800-2-chris@chris-wilson.co.uk