drm/amdgpu: split finding idle VMID into separate function

No functional change, but makes it easier to maintain the code.

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>