drm/i915: Emit await(batch) before MI_BB_START

Be consistent and ensure that we always emit the asynchronous waits
prior to issuing instructions that use the address. This ensures that if
we do emit GPU commands to do the await, they are before our use!

Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200510102431.21959-1-chris@chris-wilson.co.uk