drm/amdgpu: make VMID assignment more fair v2

Similar to finding an idle one before reuse.

This guarantees fairness between processes. Otherwise process with a reserved
VMID have an unfair advantage while scheduling jobs.

v2: improve commit message

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>