drm/amd/amdgpu: Add bank selection for MMIO debugfs (v3)

(v2) Added INSTANCE selector
(v3) Changed order of bank selectors

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>