drm/amdgpu: For Navi12 SRIOV VF, register mailbox functions

Mailbox functions and interrupts are only for Navi12 VF.

Register functions and irqs during initialization.

Reviewed-by: Emily Deng <Emily.Deng@amd.com>
Signed-off-by: Jiange Zhao <Jiange.Zhao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>