drm: Remove redundant 'flush_workqueue()' calls

'destroy_workqueue()' already drains the queue before destroying it, so
there is no need to flush it explicitly.

Remove the redundant 'flush_workqueue()' calls.

This was generated with coccinelle:

@@
expression E;
@@
- 	flush_workqueue(E);
	destroy_workqueue(E);

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Jyri Sarha <jyri.sarha@iki.fi>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Link: https://lore.kernel.org/r/75e8ba40076ad707d47e3a3670e6b23c1b8b11bc.1633874223.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Rob Clark <robdclark@chromium.org>