drm/amdgpu: restructure amdgpu_vmid_grab

Now that we have the different cases for grabbing a VMID in separate
functions, restructure the top level function to only have one place
where VMIDs are assigned to jobs.

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>