drm/amdgpu: add amdgpu_sdma_sched_mask debugfs

Userspace wants to run jobs on a specific sdma ring for verification purposes.
This debugfs entry helps to disable or enable submitting jobs to a specific ring.
This entry is populated only if there are at least two or more cores in the sdma ip.

Signed-off-by: Jesse Zhang <jesse.zhang@amd.com>
Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Tim Huang <tim.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>