Take the encode rings into account in the idle work handler.

v2: fix typo: s/num_uvd_inst/num_enc_rings/
Reviewed-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>