drm/amdgpu: power vcn 3_0 by instance

For vcn 3_0, add ip_block for each vcn instance during discovery stage.

And only powering on/off one of the vcn instance using the
instance value stored in ip_block, instead of powering on/off all
vcn instances. Modify the existing functions to use the instance value
in ip_block, and remove the original for loop for all vcn instances.

v2: rename "i"/"j" to "inst" for instance value.

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>