drm/radeon: don't add files at control minor debugfs directory

drm/radeon: don't add files at control minor debugfs directory

Since commit 8a357d10043c ("drm: Nerf DRM_CONTROL nodes"), a
struct drm_device's ->control member is always NULL.

In the case of CONFIG_DEBUG_FS=y, radeon_debugfs_add_files() accesses
->control->debugfs_root though. This results in the following Oops:

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
  IP: radeon_debugfs_add_files+0x90/0x100 [radeon]
  PGD 0
  Oops: 0000 [#1] SMP
  [...]
  Call Trace:
   ? work_on_cpu+0xb0/0xb0
   radeon_fence_driver_init+0x120/0x150 [radeon]
   si_init+0x122/0xd50 [radeon]
   ? _raw_spin_unlock_irq+0x2c/0x40
   ? device_pm_check_callbacks+0xb3/0xc0
   radeon_device_init+0x958/0xda0 [radeon]
   radeon_driver_load_kms+0x9a/0x210 [radeon]
   drm_dev_register+0xa9/0xd0 [drm]
   drm_get_pci_dev+0x9c/0x1e0 [drm]
   radeon_pci_probe+0xb8/0xe0 [radeon]
  [...]

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

Fixes: 8a357d10043c ("drm: Nerf DRM_CONTROL nodes")
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161203144700.2307-1-nicstange@gmail.com