Since commit 8a357d10 ("drm: Nerf DRM_CONTROL nodes"), a
struct drm_device's ->control member is always NULL.

In the case of CONFIG_DEBUG_FS=y, amdgpu_debugfs_add_files() accesses
->control->debugfs_root though. This results in a NULL pointer
dereference.

Fix this by omitting the drm_debugfs_create_files() call for the
control minor debugfs directory which is now non-existent anyway.

Fixes: 8a357d10 ("drm: Nerf DRM_CONTROL nodes")
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98915Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Mike Lothian <mike@fireburn.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161205203022.11671-1-nicstange@gmail.com