drm/atomic: cleanup debugfs entries on un-registering the driver.

Cleanup the debugfs entries created by
commit 6559c901:  drm/atomic: add debugfs file to dump out atomic state
when the driver's minor gets un-registered. Without it, DRM drivers
compiled as modules cannot be rmmod-ed and modprobed again.

Tested-by: Brian Starkey <brian.starkey@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20161117114129.2627-1-Liviu.Dudau@arm.com
Fixes: 6559c901 ("drm/atomic: add debugfs file to dump out atomic state")