drm/amdgpu: Doorbell layout for vega20 and future asic

This introduces new doorbell layout for vega20 and future asics

v2: Use enum definition instead of hardcoded value

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>