drm/bochs: Fix connector leak during driver unload

drm/bochs: Fix connector leak during driver unload

When unloading the bochs-drm driver, a warning message is printed by
drm_mode_config_cleanup() because a reference is still held to one of
the drm_connector structs.

Correct this by calling drm_atomic_helper_shutdown() in
bochs_pci_remove().

Fixes: 6579c39594ae ("drm/bochs: atomic: switch planes to atomic, wire up helpers.")
Signed-off-by: Sam Bobroff <sbobroff@linux.ibm.com>
Link: http://patchwork.freedesktop.org/patch/msgid/93b363ad62f4938d9ddf3e05b2a61e3f66b2dcd3.1558416473.git.sbobroff@linux.ibm.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>