drm/amdgpu: Add amdgpu_vcn_sched_mask debugfs

Add debugfs entry to enable or disable job submission to
specific vcn instances. The entry is created only when
there is more than an instance and is unified queue type.

Signed-off-by: Sathishkumar S <sathishkumar.sundararaju@amd.com>
Reviewed-by: Jesse Zhang <jesse.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>