drm/i915: Drain freed object after suspend display

Display is turned off by i915_drm_suspend() during the suspend
procedure, removing the last reference of some gem objects that were
used by display.

The issue is that those objects are only actually freed when
mm.free_work executed and that can happen very late in the suspend
process causing issues.
So here draining all freed objects released by display fixing suspend
issues.

Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Matt Atwood <matthew.s.atwood@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220629134721.48375-1-jose.souza@intel.com