drm/panfrost: Don't sync rpm suspension after mmu flushing

Lockdep warns about potential circular locking dependency of devfreq
with the fs_reclaim caused by immediate device suspension when mapping is
released by shrinker. Fix it by doing the suspension asynchronously.

Reviewed-by: Steven Price <steven.price@arm.com>
Fixes: ec7eba47 ("drm/panfrost: Rework page table flushing and runtime PM interaction")
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://lore.kernel.org/all/20230108210445.3948344-3-dmitry.osipenko@collabora.com/