drm/i915: Wait for scanout to stop when sanitizing planes

When we sanitize planes let's wait for the scanout to stop
before we let the subsequent code tear down the ggtt mappings
and whatnot. Cures an underrun on my ivb when I boot with
VT-d enabled and the BIOS fb gets thrown out due to stolen
being considered unusable with VT-d active.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210217162050.13803-1-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>