drm/amdgpu: Remove the while loop from amdgpu_job_prepare_job

While loop makes it sound like amdgpu_vmid_grab() potentially needs to be
called multiple times to produce a fence, while in reality all code paths
either return an error, assign a valid job->vmid or assign a vmid which
will be valid once the returned fence signals.

Therefore we can remove the loop to make it clear the call does not need
to be repeated.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>