drm/amdgpu: remove the check for sessions being closed

This will make clock and power gated when no block decoded, for example
when paused during the playback.

Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>