drm/amdgpu: set powergating state by vcn instance

Set powergating state by vcn instance in idle_work_handler() and
ring_begin_use() functions for vcn with multiple instances.

v2: Add instance parameter to amdgpu_device_ip_set_powergating_state(),
instead of creating new function.

Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>