drm/amdgpu/vcn: Document IRQ per-instance irq behavior for VCN 4.0.3

drm/amdgpu/vcn: Document IRQ per-instance irq behavior for VCN 4.0.3

When examining the VCN function init, it is common to find a loop that
initializes VCN rings, which uses one IRQ per instance. However, VCN
4.0.3 deviates from this pattern, as it includes a distinct field to
differentiate instances, which results in a slightly different ring
init. This commit makes this difference explicit by using a fixed index
when initializing the ring buffer and also adds a comment.

Signed-off-by: Rodrigo Siqueira <siqueira@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>