drm/vkms: Fix connector leak at the module removal

drm/vkms: Fix connector leak at the module removal

Currently, vkms shows an error message if the following steps occur: (1)
load vkms, (2) perform any specific operation in the vkms (e.g., run an
IGT test), and (3) unload the module. The following error message
emerges:

[drm:drm_mode_config_cleanup [drm]] *ERROR* connector Virtual-1 leaked!

This commit fixes this error by calling drm_atomic_helper_shutdown()
before drm_mode_config_cleanup, which turns off the whole display
pipeline and remove a reference related to any connector.

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180719004045.hzepp565x5lfco3c@smtp.gmail.com