drm/amdgpu: cleanup and simplify amdgpu_vmid_grab_reserved

Drop the "_locked" from the name, cleanup and simplify the logic a bit.
Add missing comments.

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