drm/amdgpu: use a dummy owner for sysfs triggered cleaner shaders v4

Otherwise triggering sysfs multiple times without other submissions in
between only runs the shader once.

v2: add some comment
v3: re-add missing cast
v4: squash in semicolon fix

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>