drm/amdgpu: do not keep debugfs dentry

Cleanup unnecessary debugfs dentries and surrounding functions.

v3: remove return value check for debugfs_create_file()
v2: remove ttm_debugfs_entries array.
    do not init variables.

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>