drm/amdgpu/nbio: add functions to query ras specific interrupt status

ras_controller_interrupt and err_event_interrupt are ras specific interrupts.
add functions to check their status and ack them if they are generated. both
funcitons should only be invoked in ISR when BIF ring is disabled or even not
initialized.

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