drm/ast: Fix connector leak during driver unload

When unloading the ast 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_crtc_force_disable_all() in
ast_fbdev_destroy().

Signed-off-by: Sam Bobroff <sbobroff@linux.ibm.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1e613f3c630c7bbc72e04a44b178259b9164d2f6.1543798395.git.sbobroff@linux.ibm.com