drm/amdgpu: do not use drm middle layer for debugfs

Use debugfs API directly instead of drm middle layer.

This also includes following debugfs file output changes:
1 amdgpu_evict_vram/amdgpu_evict_gtt output will not contain any braces.
  e.g. (0) --> 0
2 amdgpu_gpu_recover output will print return value of
  amdgpu_device_gpu_recover() instead of not so important "gpu recover"
  message.

v2: * checkpatch.pl: use '0444' instead of S_IRUGO.
    * remove S_IFREG from mode.
    * remove mode variable.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>