drm/cirrus: Fully embrace devm_

With the drm_device lifetime fun cleaned up there's nothing in the way
anymore to use devm_ for everything hw releated. Do it, and in the
process, throw out the entire onion unwinding.

Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: virtualization@lists.linux-foundation.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-31-daniel.vetter@ffwll.ch