drm/amd/pm: remove logically dead code

Execution cannot reach this statement: case POWER_STATE_TYPE_BALAN.

Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Acked-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>