drm/amdgpu: pull ras controller int status only when ras enabled

ras_controller_irq and athub_err_event_irq are only registered
when PCIE_BIF ras is marked as supported. as the result, the driver
also just need pull the int status in such case.

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