media: vsp1_dl: add a description for cmdpool field

Gets rid of this build warning:
	drivers/media/platform/vsp1/vsp1_dl.c:229: warning: Function parameter or member 'cmdpool' not described in 'vsp1_dl_manager'

Fixes: f3b98e3c ("media: vsp1: Provide support for extended command pools")
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>