drm/amdgpu: Add field in amdgpu_dev to hold reserved doorbell index

This is a preparation work to make reserved doorbell index per device,
instead of using a global macro definition. By doing this, we can easily
change doorbell layout for future ASICs while not affecting ASICs in
production.

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>