drm/amdgpu: adjust s2a entry register for sdma doorbell trans decoding

Use s2a entry 5/6 registers to decode sdma doorbell trans on different AIDs,
which aligns the entry table in SHUB spec, and leave entry 4 dedicated for VCN
doorbell to avoid conflict.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>