drm/cirrus: add drm_driver.release callback.

Move final cleanups from cirrus_pci_remove() to the new callback.
Add drm_atomic_helper_shutdown() call to cirrus_pci_remove().

Use drm_dev_{enter,exit,unplug} to avoid touching hardware after
device removal.

v4: add changelog.
v3: use drm_dev*.
v2: stop touching hardware after pci_remove().

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20200211135522.23654-1-kraxel@redhat.com