drm/i915/gt: Push the flush_pd before the set-context

Move our "wait for the PD load to complete" paranoia before the
MI_SET_CONTEXT just in case the context restore tries to access local
addresses.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191130120503.1609483-1-chris@chris-wilson.co.uk