drm/amdgpu: Remove unnecessary ras cap check

RAS global isr will only be invoked by hardware
interrupt. Don't need to query ras capability in isr
In addition, amdgpu_ras_interrupt_fatal_error_handler
ensures the isr won't be called from guest linux
side by accident. The RAS cap check in isr that
introduced to fix sriov crash is not needed any more

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>