drm/amdgpu: fix SI failure due to doorbells allocation

SI hardware does not have doorbells at all, however currently the code
will try to do the allocation and thus fail, makes SI AMDGPU not usable.

Fix this failure by skipping doorbells allocation when doorbells count
is zero.

Fixes: 54c30d2a ("drm/amdgpu: create kernel doorbell pages")
Reviewed-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Icenowy Zheng <uwu@icenowy.me>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>