drm/amd/pm: correct the workload setting

Correct the workload setting in order not to mix the setting
with the end user. Update the workload mask accordingly.

v2: changes as below:
1. the end user can not erase the workload from driver except default workload.
2. always shows the real highest priority workoad to the end user.
3. the real workload mask is combined with driver workload mask and end user workload mask.

v3: apply this to the other ASICs as well.
v4: simplify the code
v5: refine the code based on the review comments.

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>