drm/amdgpu: Use asic specific doorbell index instead of macro definition

ASIC specific doorbell layout is used instead of enum definition

Signed-off-by: Oak Zeng <ozeng@amd.com>
Suggested-by: Felix Kuehling <Felix.Kuehling@amd.com>
Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>