drm/amdgpu: add human readable debugfs control support (v2)

Currently, the debugfs control node can't parse bash-like commands.
Now add such support for any tester that uses scripts.

v2: squash in fixes for input validation

Signed-off-by: xinhui pan <xinhui.pan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>