drm/amdgpu: move hw generation check into amdgpu_doorbell_init v2

This way we can safely call it on SI as well.

v2: fix type in commit message

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>