drm/amdgpu/si: fix SI after doorbell rework

SI does not use doorbells, move asic doorbell init later
asic check.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=108920


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