drm/amd: Fix unbind/rebind for VCN 4.0.5

drm/amd: Fix unbind/rebind for VCN 4.0.5

Unbinding amdgpu has no problems, but binding it again leads to an
error of sysfs file already existing.  This is because it wasn't
actually cleaned up on unbind.  Add the missing cleanup step.

Fixes: 547aad32edac ("drm/amdgpu: add VCN4 ip block support")
Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>